did the engine turn over at all, or, when you turned the key, did all the dash lights/clock lights turn off?
few possibilites -
the immobiliser cable came loose on mine disabling me from starting the engine. Locate this by following the big cluster of wires leading into the ECU down towards the silvery metal plate with "JC etc" written on it, there should be some insulated wires in that weird plastic covering, give em a wiggle. - also, with regards to the immob, the car has to recognise your key when its inserted, so if you hold it into the start position for longer than you think it may start it. Mine did this yesterday, i tried to start it normally, but it only worked when i held it in the start position, as if i was turning the engine over, and then it suddenly turned the engine and worked.
Maybe its ya alternator?
Could it be the starter motor? bendicks spring?