1 ) First of all .... try with the AC original launcher ... so you know if it might not only be a Content Manager problem for some unknown reason.
2)
A) If it's working fine or much better,
uninstall ( or rather delete ) CM surely the 1st thing to do in this case
If I don't forget anything... for CM ( except the exe itself ) most or all its files are in Users/YOU/Appdata/Local ... under the name AcTools Content Manager.
B)
If it's not working better with the original launcher.... check files integrity of AC in Steam as the problem might be an original file missing or corrupted
3) Still not working afterwards with pure AC Uninstalling Assetto Corsa in Steam ... should delete all original files but maybe leave some addon files ( full or not ) .... which you should have to delete manually.
This last step should be a very radical solution to try first !!
Are you sure nothing else changed on your PC: a recent new Win10 update ? ... a recent GPU driver installed ? ... another problem in other tasks with PC ? .... maybe a problem with MSI Afterburner or similar ?
Try first to be sure it's not a problem somewhere else than AC or CM itself ....