Pontiac grand am 2001 no start

Pontiac grand am 2001 no start. Car ran fine turned off would not restart. Sat for a few days and then had the mechanic look at it. It would start but would not stay started or running. The mechanic said he thought it was the fuel pump regulator or fuel injector.

So we had it towed to a shop and where told it was the battery. But the battery was only 4 months old. They said it could be a dead cell in the battery or it was our computer system.

I am wondering if the battery was bad and we tried multiple times to start it if this could cause the security issue, they said if it happens again to take it to the dealership.