I migrated to openHAB 4.0 yesterday and everything was working quite well for some hours except the fact, that all my Things have gone now. I just see two of them (they have been added GUI based) - but everything that I’ve configured over the years in *.thing files are not visible at in the GUI.
I absolutely don’t know where to start searching for this issue so I’m grateful for every hint.
First place to look is the openhab.log file after startup…Your files should be parsed, one-by-one, right at the start of the log, and any errors the parser finds will be listed there.
Have you tried restarting the service?
Yes - I not only restarted the service but also the server itself with no difference.
Cleaning the cache and restarting seemed to solve it now for me.
Nevertheless, it is quite strange as it was running for hours after the upgrade and suddenly stopped working as intended 
First place to look is the openhab.log file after startup
Also the log looked quite normal
Update: As this behavior made me mad (and nearly EVERYTHING of my configuration is file based) I totally removed the OH4 installation and re-installed it again. Since this “special move” the problems have gone 
Maybe only cache issue. Next time clean cache after those kind of upgrade.
Sorry - forgot to mention, that I’ve tried cleaning the cache via …
sudo systemctl stop openhab
sudo openhab-cli clear-cache
sudo systemctl start openhab
…about 20 times in approximately one week - the behavior was still the same 