This may bring back system files, though it rarely recovers personal data. Prevention
The is a malicious encryption program belonging to the STOP/Djvu family . When it infects a system, it locks personal files (documents, photos, and videos) and appends the .u2k extension to them. It then leaves a "_readme.txt" note demanding a ransom, typically $490 to $980, in exchange for a decryption tool. U2K ransomware [.u2k file virus] removal.
Turn off Wi-Fi or unplug Ethernet cables. This may bring back system files, though it