Radarr has a setting which will delete files when a movie becomes unmonitored. By any chance did you recently unmonitor everything or make a change to whole directory like rescanning ?
Keep an eye out for TorrentLeech open registration period. Usually happens around Christmas, and they have a very good seeding policy and lots of freeleech content. Easily my favourite tracker.
I use a NanoPi M4 V2, which has a SATA hat accessory. It will support 4 HDDs and even power them if they are all 2.5" drives, or you can do what I did and use 4x 3.5" drives in a simple cheap cage and a separate 12V 8A power supply on the side to power the lot. No problems streaming 4K x265 content using Plex (with a compatible Plex Media Client)
Second guessing myself now but I remember finding such a setting (in addition to the reverse… Unmonitoring if deleted from disk), and thought it sounded dangerous… but it was disabled by default from memory.
One other thing is that Radarr has a recycle bin… I think 7 days retention by default. I’ve never checked it myself but maybe you can get your stuff back from there?
Of course if it’s empty then I guess Radarr didn’t delete them.