Case:

When u run below command on Principal server:

alter database 'Database_Name' set partner off

After running above command your DB goes in [mirror, disconnected] State on mirror server.

Resolution:

Step 1:Run below query on mirror

exec sp_resetstatus 'Database_Name'

Step 2: After running below command your DB will be available.

restore database 'Database_Name' with recovery

Note: Step 2 will take some time to recover.