Most likely, the User Expiration date has expired. The default time is 5 years from date of activation.
If a large number of Users stopped working at the same time, that is the first item to check.
If the user and access has expired, you will have to change the expiration for the user and add the access back to the doors. If you use the MT20W, you will have to update the fob and do a long press of the sync to the doors.
If not use MT20W, assign access and do the long press fo the sync to the door.
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