Can't access Microsft SQL Server 2005 after backup

I have deatached the database, after this i have backup a later version of the database backup. Now i can access to the database through the MSQL server 2005 with no problem, but the program that needs to connect the database fires an SQL exception, it says that can't open the database. I am afraid i have done something wrong while the backup because no changes have been madden.