Once more with feeling...
Error log entries relating to the database please. SQL will log why the DB was marked suspect. Take a read through the error log and post any error messages relating to the DB (not login failures, they are uninteresting).
What exact backups do you have, full, differential and log? I hope you have more than just a single differential.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability