The first thing I would check is the data bus connections, and it sounds like you say you did that, but did not say exactly what you did. I remove all wires from the termination screws and re-seat them and tighten the screws. Also, remove and re-seat any connectors, including the green termination blocks on the M1 (they are removable even though it is difficult to tell). With RJ45 connectors, cut the old ones off and crimp on new ones. A poor crimp connection can fail over time and be impossible to see. But before you do all that, you may want to try moving keypads around (see below).
How is the keypad connected to the M1? Directly to the data bus terminals on the M1? Daisy chained through other data bus devices? Through a M1DBH? What other devices do you have on the data bus and how are they all wired?
Are you sure your data bus terminations are correct? I've seen improper terminations work for a long time until suddenly things stop working.
It's possible that the cable between the M1 and the keypad has developed a fault. What I do is remove the keypad and connect it directly to the M1 through a short cable with no other devices on the data bus. That will eliminate the cable as a source of the problem. Note that you may have to change your termination temporarily when you do this test. If the keypad doesn't work with a short cable, the keypad may have developed an internal problem with the data bus interface.
If you have another keypad (even if you have to move it from another location) connect it up at the failing location to test the wiring back to the M1. If a different keypad works there, that would again point to the keypad as being bad.
