SPID 51 is usually a user process, but in this case, it is likely SQLAgent. Try just starting SQL Server alone without SQLAgent. If that does not work, try starting SQL Server with a minimal configuration (-f startup parameter). This should get the server up to a point where you can log in to look for problems (run DBCC commands, query DMVs, etc.), but it will not be good to run your applications on this. Are you on the latest service pack (SP4)?