Altium not starting!
This happens to me quite frequently. A couple of things to try.

Try opening the workspace directly. If you save your workspace into a specific folder, then use that workspace. On Windows 7 my workspace defaults to C:\Users\sglover\AppData\Roaming\Altium\Altium Designer {B7BBF47B-2F84-45EA-954F-607E59116D16}\LastWorkspace. Yours may be similar.

If that doesn't work then you may need to delete the workspace and all of the temporary files that Altium creates. On my Windows 7 PC these are in 2 places: 


C:\Users\sglover\AppData\Local\Altium
C:\Users\sglover\AppData\Roaming\Altium


The safest way is to rename these Altium folders to "Altium old", then try restarting Altium. You will need to "kill off" any instances of Altium that are still running by using the Task Manager first.