If you have already Rebooted and the problem still exists, you can still try this but I doubt it will help. If you haven't (and dont want to reboot for whatever reason) give it a shot. It won't hurt and may solve your problem
I have had this happen several times. While viruses or malware MAY cause this a more common problem is an app you are running that is "not playing nice with memory". It has happened to me while running something I really don't want to cancel and restart.
This solution is quite simple and DOES NOT Require a REBOOT.
1. Open Task manager (Cntl-Alt_Dlet),
2. Click Process tab,
3. Find Explorer.exe (or explorer.exe).
4. Terminate it. (ignore warning)
5. Click Applications tab
6. Then New Task.
7. Enter (guess what) explorer.exe and press enter
your desktop and taskbar will reappear (and your long running app will still be running)