Most likely one of the garage door cables has come off of the cable drum or snapped (side that is hanging the lowest) from closing on something. The cables may still be under spring tension and should not be touched without the proper knowledge and tools to make the repair. In addition, trying to force the garage door closed or open may cause further damage to the garage door tracks and may also cause the door to fall to the floor. In most cases you are better off to leave the door in whatever position it is in until it can be properly repaired.