can’t select database

“Cana€?t select database
We were able to connect to the database server (which means your username and password is okay) but not able to select the blog database.
* Are you sure it exists?
* On some systems the name of your database is prefixed with your username, so it would be like username_wordpress. Could that be the problem?”

yes that’s the problem, the databases name is domainname_database
how do I solve it? I cannot change the first part of the name, my domainname is always in front of the name I give to my database ??