Ransomware

Ransomware encrypts files or blocks access to the system, then demands payment (usually in cryptocurrency) to recover the data. On websites it can show up as a locked admin panel or content replaced with a ransom demand.

The best defence is prevention: recent backups kept off the server mean that instead of paying a ransom, you simply restore a clean version of the site. Paying a ransom never guarantees you get the data back.