There are several reasons why your garage door may not open. The first possibility is you could have a broken spring. If the torsion spring (this spring sits above your garage door with a metal rod running though it) or an extension spring (these springs are to the side of the door near the tracks, one on the left and one on the right) breaks, it removes all the tension off of the door causing it to become extremely heavy, or unable to lift at all. Trying to repair this yourself can be dangerous and is not advised! Please call & schedule an appointment.
Also, check the cables on both sides of the door to be sure that they are not broken or frayed.
Another reason could be that the sensitivity is out of adjustment on your garage door opener. This can happen to a newly installed opener as well as an older opener. You can test you sensitivity as per the manufactures directions. This should be done on a regular basis.