The mechanism is quite primative. As the door is open, you won't be able to easily see the spring/cables. It could be one of the cables has jammed - they wind round grooved connicle spindles and can tangle if the door goes up too quickly. If the cables aren't jammed, my guess would be it's jammed in the runners.
I had the same problem with my garage door a few years back. A lack of oiling on my part caused the spring and complete mechanism to seise. It cost me a pretty penny to get it replaced - if only I had oiled it regularly all would have been fine. A valuable (and expensive) lesson.
Usually you can release the spring(be very careful!) and unwind/untangle the wires. You can then retension the spring(requires a couple of spanners). The instructions are available on the net or sometimes are supplied with the door. However, only try this if you are reasonably handy as the spring could injure you.
These doors should ideally be lubricated every 6 months.
I have had to do this with a couple of doors and have successfully recovered them at no cost either financially or physical!
If you do not feel confident then the manufactures will send out an engineer.
If it's a Henderson type arrangement, you don't need to untension the spring to replace the cables. With the door closed, you just need to turn the spring by a quarter turn and insert a large nail, or similar, in the hole provided. That's enough to release the tension on the wires.
Replacement doesn't have to be expensive. Wickes do them for £350. Ours was made by Wessex Doors although Wickes supplier may have changed. Easy DIY job. I fitted one about 7 or 8 years ago and it works well with little maintenance. We use the garage every day.
I did this 3 weeks ago! Shutting the door it hit something on the way down and jammed open.
I called a garage specialist who visited me later the same day - the metal guide wire on one side had become entangled. He also serviced the door (WD40 on the metal wire helps keep it sweet). Cost was not too bad, £40, with a receipt - not cash in hand!