Well you may not actually see any error in the SQL ERRORLOG file but default location is:
C:\Program Files\Microsoft SQL Server\MSSQ.1\MSSQL\LOG.
I would actually first look in your Windows Event Log (Start > Run > eventvwr). The Application and/or System Log should also tell you why it did not start, or what issue it is caught on.
Is the service set to run as a system account (Local System, or system account like NT AUTHORITY\SYSTEM) or a local/domain account? This can effect permissions if it is running as a customized account, depending on how it was setup.
If it is a local account within the properties of the service (using SQL Server Service Manager) re-enter the usrename and password. This will attempt to give it the correct permissions it requires.