Cable has probably just slipped, it will most likely need to be tightened/readjusted. In the meantime just get good with your clutch and get off the brakes and onto it quickly on the hill starts! ;) :P
If you use it frequently, the handbrake, and require a lot of pressure on steep hills to keep it still it may be worth fitting a heavy duty cable to the thing; will last longer, not slip as easily and be stronger and not snap.
Shouldn't be anything wrong with the brakes; unless they are very badly worn?
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