This is normally associated with malware so it will be adviseable to get a strong scanner and let it fix anything it finds.
Firstly, press Control Shift and Escape simultaneously and you'll bring up the Task Manager. Click the Processes tab and from the File menu, select New Process then into the box type exlorer.exe and hit Enter. That should bring the desktop back.
I suggest you then go top http://www.malwarebytes.com and download MalwareBytes. Install and update it then restart your computer into Safe Mode with Networking by tapping the Function 8 key before Windows starts to load. Click the MalwareBytes icon and then on Full Scan. When it completes click Show Results and then Remove Selected and restart the system back into Normal Mode.
If you can't get past the first stage of this - calling up Task Manager, go straight to the Safe Mode bit and download MalwareBytes.