How do I undo a change that broke my computer?

There is a ladder of undo mechanisms, narrowest first. Using them in order fixes most self-inflicted problems without reinstalling anything.

Difficulty: intermediate
Time: 45 min
Read: 4 min
Safety: warning

Short answer

Work from the narrowest undo to the widest. Uninstall or roll back the specific driver or program if you know what changed. Then a system restore point on Windows, which reverts system files and settings but leaves your documents. Then uninstalling the recent update, which Windows and macOS both allow for a limited period after installation. Then restoring from a full backup. Reinstalling the system is the last rung, not the first.

Do this first

How do I start Windows or a Mac in safe mode, and what does it tell me?

Safety

Every undo mechanism here rolls the system back to an earlier state, and some of them also roll back programs installed since. None of them is a backup of your documents, and a restore point is not a substitute for one. If the machine is stable enough to copy your files off, do that before you roll anything back.

Step by step

Establish what changed, and when.Windows' reliability monitor shows a timeline of installations, updates and failures side by side, which is often enough to name the culprit in a minute. On a Mac, the installation history in the system information tool serves the same purpose.
Rung one: undo the specific thing.Uninstall the program, or use the driver's roll-back option to return to the previous version. This is precise, quick and reversible, and it is the right first move whenever you know what changed.
Rung two: a system restore point on Windows.Restore points capture system files, drivers, the registry and installed programs at a moment in time. Restoring returns those to how they were without touching your documents. Programs installed since the point are removed, so note what you will lose.
Check restore is even switched on.System protection is not enabled on every machine, and if it was off there will be no points. Turn it on now if it is off — it costs a modest amount of disk space and is worth having ready before the next problem.
Rung three: uninstall the update.Windows lets you remove recent quality updates, and lets you go back to the previous major version within a limited window of days after upgrading. macOS does not roll back a system update in place, so the equivalent is restoring from Time Machine.
Rung four: restore from your backup.A Time Machine restore, or a system image restore, returns the whole machine to a known good moment including your files. This is why the image half of a backup exists, and it is the point at which having one saves a day.
Use safe mode when the change prevents starting.If the machine will not reach the desktop, the recovery environment reached after failed starts gives access to restore points, update removal and safe mode. Windows opens it automatically after repeated failures; on a Mac, the recovery start-up options do the same.
Rung five: reset or reinstall.The last rung, and it is a day's work with the reinstallation of programs afterwards. Take it when the earlier rungs have failed and the machine is genuinely unusable, not because it is the advice you found first.
Make the next one easier.Before the next major update or driver change, create a restore point manually and take an image. Both take minutes and turn a lost weekend into a lost half-hour.

If it doesn't work

Machine is unstable after a driver update

Cause: The new driver is faulty or wrong for the hardware — Fix: Roll the driver back in the device's properties, then prevent it from being reinstalled automatically until a newer version is out. Get replacement drivers from the machine or component manufacturer.

Something broke after a major system upgrade

Cause: Compatibility with an older program, driver or peripheral — Fix: Check whether the version-rollback window is still open — it is a limited number of days. Beyond that, look for updated versions of the software that broke; a major upgrade often simply ends support for an old program.

No restore points exist

Cause: System protection was disabled — Fix: Nothing to restore to now. Enable it, then rely on update removal or a backup for this incident.

Restore fails partway and reverts

Cause: Antivirus interfering, or a damaged restore point — Fix: Try again from safe mode, which is the recommended environment when a normal restore fails. If it fails repeatedly, move to update removal or the backup.

Machine will not start at all after a change

Cause: A boot-critical driver or system file — Fix: Let it fail two or three times so the recovery environment appears, then use safe mode, the restore point, or update removal from there.

Questions people ask

Does a system restore delete my documents?

It is designed not to, and it leaves personal files alone. It does remove programs installed after the point. Do not use it as a file recovery tool — it will not bring back a deleted document.

How far back can I go?

As far as the oldest restore point kept, which depends on how much space is allocated and how often points are created. Typically weeks rather than months. The update rollback window is a fixed number of days after the upgrade.

Is there a Mac equivalent of restore points?

Time Machine's local snapshots are the closest thing and serve for files and, with a full restore, for the system. macOS does not have an in-place system rollback in the Windows sense, which makes a working Time Machine backup more important, not less.

Should I turn off automatic updates to avoid this?

No. Updates fix security holes that are exploited far more often than updates break machines. The better answer is to take a restore point and an image before major upgrades, and to delay optional updates by a short period rather than avoiding them.