circuit board

My waters 510 HPLC pump worked fine until last time I turned it on. Only the fan worked, otherwise no response. I opened the cover and saw there are four indicator leds in the circuit board. Only one of the leds "a" was light up. I checked the four big electrolytic capasitors but they seem to work. Could you give me any tips how to proceed? Which could be most likely the faulty component? What are the indicator leds telling for?