Turn off Wi-Fi or unplug Ethernet cables.
To prevent the ransomware from spreading to cloud storage or networked drives: U2K ransomware [.u2k file virus] removal.
This is the most difficult part. There are two scenarios for STOP/Djvu variants: Turn off Wi-Fi or unplug Ethernet cables
If the virus didn’t delete your Shadow Copy backups, this tool can revert files to their previous state. U2K ransomware [.u2k file virus] removal.
Tools like PhotoRec or Recuva can sometimes find "deleted" original versions of files that were swapped for encrypted ones.
Use reputable tools like Malwarebytes, Emsisoft, or Windows Defender. These programs are effective at identifying and deleting the core STOP/Djvu registry keys and executable files.
Removing the .u2k extension from a file won't fix it; you must first remove the "engine" that encrypted it.