After trying to startup the instance examine the errorlog to find out more details about the problem. Current errorlog can be typically found in directory like:
<basedrive>:\Program Files\Microsoft SQL Server\MSSQL10_50.<instancename>\MSSQL\Log
and is named
ERRORLOG
without any extension. The file itself is a textfile so it can be opened for example with notepad.
From this file you should be able to find the source of the problem.