If your computer does not seem to get past the grey screen, attempt a startup in Safe Mode.
http://support.apple.com/kb/HT1455
Basically, starting up the computer and holding down the "Shift" key. This will force a file system check of the operating system, and will in most cases fix any kind of file corruption that might be causing the stall. This will also clear out some caches. And the startup will take a bit longer than usual.
After this is done, just restart the computer again after it is successful.