Not starting

After running whole day, it parked my vehicle it is on a bit off a slant nothing big. Tried starting it again after a few hour of it sitting. It would crank but it would not turn over, may I add it is low on fuel but it usually would start even if the gas light is on. So I find this weird. I recently changed out my fuel pump, I would notice the rpm dropping below 500 a few times but it never stalled, this is the first time it has trouble starting hopefully it just needs some gas.