To fix a ThyssenKrupp TAC-20 elevator controller, first ensure that the power supply is stable and check for any blown fuses or tripped breakers. Next, inspect the controller for error codes or diagnostic messages that can indicate specific issues. If necessary, reset the controller by power cycling it, and refer to the technical manual for troubleshooting steps aligned with the identified errors. If problems persist, it's advisable to contact a certified elevator technician for a thorough inspection and repair.