This is a tough one because it's a microcontroller, so I'm guessing it's a reset issue. If you know where the voltage regulators are located, test the output voltage (usually 5 or 12 volts, sometimes one each). The reset of the controller (Pin 1) requires a reference of 5 volts. Check that. Then work back on the circuit trace until you see the first electrolytic capacitor connected between Pin 1 and ground. Check the cap. Then do the same for Pin 26 (VDD). This pin should be 12V (maybe 5, don't know without a schematic). Check along the circuit trace to the first electrolytic to ground. If these caps are dried out the controller will have difficulty maintaining a constant voltage.
That's just the tip of the iceberg. Then there are components that act on the controller outputs that may have failed, such as opto-couplers and transistors.