2000 Questions for all other cars

Car has been running in perfect condition, just got an oil change 2 weeks ago, they said that everything seems to be running really good w/ the car until yesterday when my hubby dropped me off to work then he went to work and tried to start the car later on. It didn't sart, it was n't turning over wasn't an electrical problem b/c we had just changed the battery 3 months ago.
so he waited till the end of the day and it started. When we got home we were about to leave the house to pick up my step-son and the car again didn't work. Now this morning it started.
my hubby thinks itsnot the fuel filter b/c it doesn't make that sound that it would do. What do you think this can be? Maybe fuel pump? Or maybe cheap gas he put in yesterday? Help!