This happened to me running SQL2000 under VMWare, running a backup with RedGate HyperBac 5.7.0.155 as the transparent compression. The backup had hung (which has happened twice on this, our only virtual SQL Server) and when I restarted MSSQLServer, Hyperbac still had a lock on master.mdf. I had to kill Hyperbac thru taskmgr (its UI didn't kill it) and then was able to restart MSSQL.