The pump has lost it's prime. There's a chance you've also sucked debris up into the screens in the mixing chamber, so you may need to drop the tank soon and either clean the screens or do the intank mods.

To answer your question, filling the tank more than half way helps. Sometimes filling the fuel filter bowl (and replacing the lid) and cycling the key will help. Don't waste the batteries trying to start. The fuel pump is electric so turn the key on and listen to the pump. It will come on for 25 seconds, then shut off. Just cycle the key on for 25 seconds, then off, then back on again. You'll be able to hear the change in sound of the pump when it's primed.

Other options would be to disconnect the fuel line from the back side of the pump, stick a short hose on the pump and dip the other end into a container of fuel. Cycle the key several times to prime the fuel lines, then reconnect the factory line and cycle the key several more times. You can also blow compressed air at about 20 psi into the tank through the fuel filler neck. Wrap a rag around the blow nozzle to help pressurize the tank some and cycle the key. Don't make an airtight seal and don't over pressurize the plastic tank.