Fix/WorkAround/Solution: Try all the suggestions one by one.
Suggestion 1: Make sure that on Mirror Server the database is restored with NO RECOVERY option (This is the most common problem).
Suggestion 2: Make sure that from Principal the latest LOG backup is restored to mirror server. (Attempt this one more time even though the full backup has been restored recently).
Suggestion 3: Check if you can telnet to your ports using command TELNET ServerName Ports like “telnet SQLServerName 5023”.
Suggestion 4: Make sure your firewall is turned off.
Suggestion 5: Verify that the endpoints are started on the partners by using the state or state_desc column the of the sys.database_mirroring_endpoints catalog view. You can start end point by executing an ALTER ENDPOINT statement.
Suggestion 6: Try the following command as one of the last options.
GRANT CONNECT ON ENDPOINT::Mirroring TO ALL
Suggestion 7: Delete the end points and recreate them.