HT1492: Mac OS X: How to start up in single-user or verbose modeLearn about Mac OS X: How to start up in single-user or verbose mode
Currently Being ModeratedNov 25, 2012 12:01 AM (in response to MACF8)
The link at the top of this page talks about the options for booting in single-user or verbose mode. These might give you a clue as to what's hanging the startup process, and may offer a chance to fix.
There are a lot of possibile causes, though, so it may take some trial and error to fix your system. The logs can be hard to interpret.