I had a similar issue today, almost all content was gone.
The pre-launcher window got stuck at "Downloading Workshop items".
When I closed it manually, the launcher started and all seemed to be right.
Except, in my content inventory, all the S397 GT3 cars were listed with one version entry only, previously they always had two entries.
So I closed the launcher and verified the integrity of game files.
I was told that one file would have been downloaded, but the progress bar at the bottom of the Steam UI, was still almost empty.
Clicking it showed that rF2 started to download 35GB.
After a few hours, download completed without errors.
When starting the launcher, I saw in the pre-launcher window that two tracks were getting updated, but after their download appeared to have been finished, it was stuck "Downloading Workshop items" mode again.
After manually closing it, my content inventory was almost empty.
Only about three or four tracks and a couple of car mods were left over.
I quit rF2, rolled back a former file backup, restarted rF2, got stuck at the pre-launcher window again, closed it, content inventory seems to be back at "normal" state.
Something's fishy, if you'd ask me...
