There will be messages in the SQL error log saying why the DB was marked suspect. Have a look through the error log, look for messages relating to the database, login failures are uninteresting and useless in identifying the cause.
I didn't ask what you're using for your backups. What backups of this database do you have?
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