What does it say in the SQL Server ERRORLOG and the SQL Agent Log?
By default they are in ...\MSSQL.1\MSSQL\LOG (initial bit depends on drive, folder where you installed SQL Server) for a default instance.
MARCUS. Why dost thou laugh? It fits not with this hour.
TITUS. Why, I have not another tear to shed;
--Titus Andronicus, William Shakespeare