How to fix a garage door that won't go down?

My garage door has worked well since I moved into my house 2 years ago, but now I'm having a problem with it. The other day I opened it and when I tried to put it down the door would go about 1/4 of the way then go back up. There's nothing blocking the sensors, but one of the lights on the sensor is red, while the other is green. I had to take the door off of the chain and manually put the door down. I'd rather try to fix it myself, as my garage door repairman charges $14O just to look at the door.