Did you accidentally dial yourself out of the group? we see this a lot when there is a local number that starts with 53 and people forget to dial 9 to call out.
To check, (if you don't know the incoming group #) go to a phone that is ringing and see what the display says, ie Call for 501 or 500 most likely.
Go back to your phone and dial 53-501(replace 501 with the correct group) and then press 1. It should give you a confirmation on the display that you will now receive group calls.
If it tells you, you are not a group member you completely lost your association with the group and will need to enter tech level programming to fix.