It could be any of a million things, unfortunately.
Things to check are:
Do a check of your hard disk. (Go to Start Menu / Programs/Accessories => Command Prompt. In the black box that appears, type
chkdsk c: /f
Press Enter to start the program. Windows will complain that it can't do it right now and ask if you want it to check when you retsart your PC. Tell it Y
Restart your PC.
Other than that, it is is likely that you have a registry error, or some program or update has become screwed up. It could be a conflict between two programs. Other possibilities include viruses and trojans. But there are so many possibilities.
You might want to try using an earlier System Restore to see if that fixes it. Go back to the time when it was working OK and restore to that. You will loose any programs you've installed since that time.
I'll move this to Small Talk. You may well get more help there, as this isn't a Ship Simulator related problem.