It seems that you have either: modified the parking position, so the jetway can not reach the door, or you have accidently switched the doors or changed something at the door config, so the jetway can not reacht the position of that door. It must be a problem with either door or parking position. see if you have a aircraft config file you don't need or if you have moved the parking to far away from the jetway.
(if you have an aircraft gsx file, you can delete it, because the PMDG 777 is supported natively and should not need any configuration file).
