It could be a couple of things...but it's not because you've been bad ~
1. They have switched their profile to not accept PM's or emails any longer - could be only temporary while they are traveling or for whatever reason. Regardless, this is a global setting not one that picks & chooses members.
2. They have put you on their 'Ignore' list for some reason...not likely though, most members don't use this feature.
3. Their mailbox could be full - each member is only allowed so much space on the server...the most likely scenario.
Note to all: Please remember to empty both your PM 'Inbox' & 'Outbox' occaisionally.