Most garage door openers (1990 and above) have sensors also known as a safety beam. You will find them at the bottom of each track of the garage door. Their job is to add an extra safety layer to the garage door system and prevent the door from shutting on something as it passes through.
There are a few reasons why a garage door will go up but won't come down. One of them is misaligned sensors. On both sensors there is a small light, and both always need to be lit. If there is no light on one of them, the door will not go down. If the lights aren't on, it's usually because of misalignment. If this is the case, all you have to do is move the sensors so they are aligned with each other.
If there is still no light, you will need to check the whole wire that connects the sensor to the opener. In some cases, the wire has been nicked somewhere and the wire needs to be replaced or reconnected. If the cable is okay and the sensors are aligned but you still don't have any light, it's probably due to a bad sensor.
If there is still no light, the sensors needs to be replaced. With some manufacturers like LiftMaster, Chamberlain and Craftsman , the opener will blink 10 times when you have a sensor issue.
* In most openers, if you push nonstop on the wall button, the door will go down even if you have a sensors problem.