So, I'm running a server with a couple of friends of mine, normally we play on vanilla 1.14.4 however i switched over to a modded server, after backing up my server files.
After switching back, the world loads, however my (along with everyone else's) inventory is completely cleared, as if i started again. I checked the playerdata folder for my UUID, and it was there, along with everyone else's, however the server seems to be having trouble detecting it? I did make sure online mode was switched on.
The server may have corrupted your playerdata upon loading the new mods, try replacing the backed up data from world/playerdata into the new world/playerdata and you should still have your inventory, make sure to keep a copy unless something like this happens.