TL;DR: Got malware? Get Malwarebytes

The fix only. No explanation, no diagnosis. Read the full article if any of this does not match what you are seeing.

If files are renamed or encrypted, or there is a ransom note: stop. Unplug the network and get help. Do not run a cleanup over ransomware.

Work computer? Call your IT or security team instead of cleaning it yourself.

1. Stop using the PC for anything sensitive. No banking, no email, no password manager.
2. Download Malwarebytes from malwarebytes.com only. If the PC blocks it, download on another machine and copy it over on a USB stick.
3. Install, let it update, run the default Threat Scan.
4. Review the results, untick anything you know is legitimate, quarantine the rest, and restart when it asks.
5. Scan again after the restart. You want a clean result before moving on.
6. Get a second opinion with a different engine: Microsoft Safety Scanner, ESET Online Scanner or AdwCleaner.
7. Remove browser extensions you do not recognise and reset the homepage and default search engine.
8. Run Windows Update until it finds nothing, and update every browser.
9. Change your passwords from a different, clean device, starting with email and banking, and turn on multi-factor authentication.

Malware blocking the scan? Restart in Safe Mode with Networking (hold Shift while clicking Restart, then Troubleshoot > Advanced options > Startup Settings) and scan from there.

← Full article, with the diagnosis and the why