This could be one of two things. If you apply pressure upwards when the door is lifting, will it go up all the way? This problem is more than likely due to a problem with a torsion spring. For most doors, torsion springs can be replaced at an affordable cost.
Otherwise, you could be looking at a problem with the motor and/or garage door. As garage doors age, they require maintenance to keep them evenly balanced. An opening garage door can find itself getting stuck in a stationery position because an usually large amount of force has been detected required to open the door.
Usually, a garage door service will mend any problems seen. Call a licensed, bonded and insured garage door professional.