What do I do if I have forgotten my computer login password?

The route depends entirely on which kind of account you have. How to tell a Microsoft account from a local one, and what each recovery path costs you.

Difficulty: beginner
Time: 30 min
Read: 5 min
Safety: caution

Short answer

If you sign into Windows with an email address, it is a Microsoft account and you reset the password online from any device — that is the easy case. A local account with no linked email is much harder and may mean resetting the machine. On a Mac, an Apple Account can reset it, and FileVault means you need the recovery key. Find your drive encryption recovery key before attempting anything that resets.

The single most useful thing to know here is that "forgot my computer password" is not one problem. A Windows machine signed in with a Microsoft account, a Windows machine with a purely local account, a Mac with an Apple Account, and a Mac with FileVault enabled are four different situations with four different answers, and one of them has no good answer at all. Working out which you have before trying anything is what prevents the outcome people most regret, which is a reset that leaves the machine working and every file inaccessible.

Safety

Only do this on a computer you own or are authorised to access. Some recovery routes reset the account rather than recovering it, which can permanently lose access to encrypted files and saved passwords. Read what a method does before running it, and make sure you have the encryption recovery key for the drive before resetting anything.

Step by step

Work out which kind of account you have.On the Windows sign-in screen, an email address under your name means a Microsoft account. A name with no email means a local account. On a Mac, the login screen usually offers to reset using your Apple Account if one is linked.
For a Microsoft account, reset it online.Go to Microsoft's account recovery page from a phone or another computer and reset the password using your recovery email, phone or authenticator. Then sign in on the computer with the new password. The machine needs a network connection at the sign-in screen, which is why the wifi icon is there.
For a local Windows account, try the security questions.Local accounts created on recent versions of Windows have three security questions set at creation. Entering the wrong password once brings up a reset password link that asks them. Many people set these without remembering that they did.
For a Mac, use the Apple Account or the recovery key.After a few failed attempts, macOS offers to reset the password using your Apple Account. If FileVault is on and no Apple Account is linked, you need the FileVault recovery key that was displayed when encryption was switched on. Without either, the data is not recoverable.
Find your encryption recovery key before doing anything drastic.On Windows, BitLocker keys are commonly saved to your Microsoft account and can be retrieved from Microsoft's device recovery key page from another device. On a Mac, the FileVault key may have been stored in your Apple Account. Retrieve it first, because a reset without it can make the data permanently unreadable.
Understand what a reset actually does.Resetting a local account password through recovery media does not decrypt an encrypted drive, and resetting the account can lose access to files encrypted to that account and to credentials saved in it. A reset that gets you a working login and an empty machine is not a success.
Get the data off before resetting, if you can.If the drive is not encrypted, a repair shop can usually copy your files off before any reset. If it is encrypted and you have no key, they cannot — encryption is doing exactly what it was designed to do.
Set up recovery properly once you are back in.Link a Microsoft or Apple Account, record the encryption recovery key somewhere outside the machine, and put the password in a password manager. This is the step that stops it happening twice.

Common mistakes

Downloading a password-cracking or bypass tool — Most are malware, several damage the account database, and none of them decrypt an encrypted drive. They also do nothing about a Microsoft account, which is authenticated online.
Resetting before locating the encryption recovery key — It is the irreversible step. With the key you have options; without it, an encrypted drive stays encrypted forever.

If it doesn't work

Sign-in screen shows an email address

Cause: Microsoft account — Fix: The easy case. Reset online from another device and sign in. Make sure the computer has a network connection at the sign-in screen.

PIN not working but you know the password

Cause: PIN is device-specific and can be reset separately — Fix: Use the sign-in options to switch to password, then set a new PIN once you are in. A PIN problem is not a password problem.

Local account with no security questions and no reset disk

Cause: The genuinely difficult case — Fix: There is no supported recovery. The realistic options are a reset install, which loses installed programs and possibly data, or professional help. Check for the encryption key first.

Mac asks for a recovery key you have never seen

Cause: FileVault enabled with the key stored elsewhere — Fix: Check your Apple Account, and check whether an employer manages the machine, in which case their IT department holds it.

Machine belongs to an employer

Cause: Managed device — Fix: Contact IT rather than attempting recovery yourself. Managed devices have policies that can wipe them on failed attempts, and IT can usually reset it in minutes.

Someone else set the machine up and is not contactable

Cause: Unknown account type and unknown recovery options — Fix: Start by identifying the account type at the sign-in screen, then check whether the email shown has a recovery route you control.

Questions people ask

Can a repair shop get into it?

For an unencrypted drive, usually yes — they can remove the drive and copy your files off, then reset the machine. For an encrypted drive with no recovery key, no, and any shop promising otherwise is either misunderstanding or misleading you.

Will resetting the password lose my files?

For a Microsoft or Apple Account reset, no. For a local account reset through recovery tools, you keep the files but may lose access to anything encrypted with that account and to saved browser and app credentials. For a full reset install, yes.

How do I avoid this next time?

Use a password manager, link a Microsoft or Apple Account so an online reset is possible, store the drive encryption recovery key somewhere physical and outside the machine, and keep a current backup so a reset is an inconvenience rather than a disaster.