Moving site to new server - admin area not working
I'd start with debugging the control panel. If you enable devMode, and load the dashboard, you might see any error message that display on the screen that would give you a clue what's going on.

If it's not there, then some error message is probably being logged in your craft/storage/runtime/logs files that would help.  Open that files and do a search for [error] and look for any suspicious messages.