Need a bit more detail on how it's broken really.
Does it not light at all (No spark from the ignition)?
If so then it's the ignition refered to by peccavi above.
If it lights, but goes out when you try to turn it up to full, then it's very likely the thermocouple.
This is quite a handy page http://patioheaters4u.com/PatioHeaterTopRepairTips.htm
Greg.
Greg, November 2009