Failing over database mirroring will swap the roles of the mirror and principal databases. The mirror database will become ...

Failing over database mirroring will swap the roles of the mirror and principal databases. The mirror database will become the principal database, and the current principal database will become inaccessible. If you have just modified any properties in the Database Properties dialog box, those changes will be lost. In addition, SQL Server must close all other connections to the current principal database. 

Are you sure you want to fail over database mirroring to the mirror database and close all connections to the current principal database?