DIAGNOSIS The learn button/LED module itself has likely failed, or it's not getting power from the logic board. This is a known weak point on this unit.
FIX You'll need to replace the learn button assembly. It's a small, rectangular plastic module with a wire harness that plugs into the main logic board. Unplug the opener. Remove the light lens cover. You'll see the module mounted near the front. Note how the wires are routed, unplug the harness, unscrew or unclip the old module, and install the new one. The part is often listed as the "wireless keypad receiver" or "learn button assembly" for the B4545T.
IF THAT DIDN'T WORK If a new learn button module still shows no life, the issue is on the main logic board. The port that provides power to the learn circuit may have failed, which would require a full logic board replacement—a much more expensive fix.
HEADS UP While you have the cover off, be very careful not to touch the large capacitor on the logic board, even with the unit unplugged. It can hold a dangerous charge. Just work around it.