It sounds like you've gotten some form of malware, but if you can boot to Safe Mode you could copy any data, then try to scan for malware. Otherwise you'll need to boot from something like Linux to save your data, then restore to factory settings.
Hi, so recently my windows 8.1 pc has been acting very strange. I left it alone for a bit only to come back to a blank screen where I can see/ move the cursor, but do nothing else. After restarting my computer and running it, I noticed it was running more slow than usual. Then suddenly after usuing it it seems all windows had closed, all icons disappeared, then the screen turned to the colour of my tile background but showed nothing, almost like when shutting down, except it did nothing and I had to forcefully shut down. I got a bunch of errors like "cannot find registered JIT" or "Explorer has crashed". I tried refreshing my pc and at the end the refresh failed and I got an error report "CRITICAL_PROSCESS_DIED". I cannot system restore either because it says no images can be found. Now the computer seems to be doing the same thing of all the programs crashing and going into a blank useless screen.