Sounds like exchange 2003,
If so check the following,
Make sure that the Database name and server name is exactly the same as it was,
Go on the the DB properties before mounting and check the box that the DB can be overwritten by a restore, otherwise it wont mount at all,
If the name of the server or db changed, then that needs to be changes on the schema,
Please let me know if these 2 steps didnt work, and then provide a little more info?