In certain scenarios, a mirror will break after fail-over and the mirror endpoints need to be restarted: https://support.microsoft.com/en-us/kb/2490051
The easiest fix is to use the following on both primary and secondary:
--get the name of your mirror endpoints select name from sys.database_mirroring_endpoints -- stop and start them ALTER ENDPOINT Endpoint_Mirroring STATE=STOPPED; GO ALTER ENDPOINT Endpoint_Mirroring STATE=STARTED; GO