Maybe the destination folder is gone?
Try this:
Right click on the "Documents" library, choose "properties" (the bottom option in the right-click menu, anyway). There, check the library locations.
If you can't access the context menu either, go to C:\Users\*Your name*\ and see if the "My Documents" folder exists. That's the folder the "Documents" library links to by default.
If all of this doesn't work, right-click "Libraries" and choose "Restore default libraries".
I'm sorry if I got some of the item names wrong, but I don't use an English version.