I needed to replace a headlight bulb

I needed to replace a headlight bulb. He was able to replace the bulbs fairly quickly, I paid and attempted to leave. The car wouldnt start! It cranks and cranks but wont start. Mechanic says its not getting fire to the engine. He seems to think its the security. Is there anything that can be disturbed while changing a simple bulb? Has anyone ever had this happen to them. We have taken off the cables in hopes to reset it but that didnt work either. Help!