Replaced head gasket and timing belt?

did you make sure you lined up the timing marks perfectly? also, check and ensure you plugged all vacuum lines and electrical connectors back in to the right spots. If you didnt put the timing cover back on i can ume you also didnt re-install the water pump alternator belts ect. since they go on after the timing cover, maybe there is the problem. Ive seen cars (all samll 4 cylinder engines) that wouldnt start simply because the V belts werent on. I dont know why (maybe the computer knew something was wrong and put the car in safe mode, maybe the cars needed the extra juice from the alternator to get the motor going) but it does happen, some cars need the belts operating properly to start.