Sounds like it's infected by ransomware, similar to
CTB-Locker from a few years ago. Check if you have any restore points from before you noticed the problem. Assuming you're using Windows 10, go to All Settings > System > About, click on System info in the top right under Related Settings, then click System protection and it'll open System Properties. Click the System Restore button and go through the wizard to check whether there are any restore points you can use. Even if there are, it's not a 100% guarantee it'll recover the files so hopefully you have backups.
UPDATE: Just saw
this article which says it's ransomware.