Reconnect mirroring server SQL Server 2008 R2
Sometimes restarting the mirroring endpoint on the principal server will resolve this.

To do this use the following T-SQL.

ALTER ENDPOINT Endpoint_Name STATE = STOPPED

ALTER ENDPOINT Endpoint_Name STATE = STARTED


To get the name of the endpoint.

SELECT * FROM sys.endpoints
WHERE type = 4


See KB 2490051 for more details.