As there is no witess available, so when resumes the mirrored database by
ALTER DATABASE <database_name> SET PARTNER RESUME;
does not resumes the mirroring. Resume button is also disabled. I don't get any error. What is the checkpoint?
Did you also check on the SQL errors? What does it says...?
One thing you can try is to restart your endpoint mirroring on both the primary and secondary instance to see if it helps.
(Stop the endpoint mirroring first)
ALTER ENDPOINT STATE=STOPPED
(Then start it back up)
ALTER ENDPOINT STATE=STARTED