If you had three reds and three greens then the most likely issue is the microswitch in the landing gear handle is not made. Long time B733 pilots always bump the gear lever in after placing the gear down to ensure the micro switch is made. Easy to see what it looks like is to pull the landing gear handle out about a 1/4 inch when it is down and you will get the reds and the greens. Just push the handle in it goes back to three greens.
The MC in the classic is not connected to the landing gear lever indications as the warning lights are in the pilot field of view.
Chapter 15 in FCOM2 The warnings section list the related systems and amber lights that activate the MC.
The blurb from the FCOM.
Two Master Caution lights illuminate when any caution occurs outside the normal field of vision of the flight crew.