2001 Ford Expedition has been parked for 6 months

Supposing it has gas and the needle is correct it could be a bad fuel pump clogged filter or a fuel pump relay.

Check the fuel pump, have another person turn the key to the on position in the ignition while you listen for a sound coming from the fuel tank, it will not last too long. If the pump is not working, you need to replace it first, if it works then check the relay; in most cases the fuel pump relay fails due to moisture collected because the vehicle hasn't been moving. If the relay is good, then replace the fuel filter, residue created by fuel clogs the filter. Even if you have to replace the fuel pump, get a new fuel filter and replace it at the same time.