Old Liftmaster will not close properly

Hi all, 

I have a 20 year old what I believe is a Liftmaster garage door closer. Over the weekend my send sensor got knocked out of alignment so I had to readjust it but after doing so and making sure all the green lights are on the door will not close without me having to hold on the wired door button. If I try to close the door with either of my wireless remotes the light on the closer will just blink and the door will not close. The door will open with my wireless remotes but will not close.

What can I try to do before I call a repairman to come fix the problem. 

Thanks

Make sure the Sensor lights are both working on each of the sensors.. Both Green or one is green and one is red. Its' still the eye sensors giving you trouble. You said you knocked it out of alignment. Clean the lens on both of them. If they are installed with wire nuts. Tighten up the wire nuts or if there is enough to cut and splice the wiring from the head to the sensor eyes. Redo them. Some times(rarely) people kick the sensors where the wire goes into the eye itself and could be broken on the inside. I am a Liftmaster/Chamberlain Dealer in Texas. I wouldn't waste a lot of time if you can't get it fixed with the advice above. But wait for someone else to give you advice as well. My experience with this matter has been rare.