Did it ever work? When did you install 2011? Is this a network or stand-alone license? Is this a Student version? Also - What OS on your computer?
Without any further clue. The best I can recommend is to completely uninstall the program http://knowledge.autodesk.com/support/autocad/trou
Start with some of our most frequented solutions or visit the Installation and Licensing Forum to get help installing your software.