DIAGNOSIS Running completely out of gas often lets air into the fuel line and carburetor. The engine has fuel in the tank, but the carburetor bowl is empty, so it has nothing to burn.
FIX You need to prime the fuel system. First, make sure the fuel valve is ON. Then, find the primer bulb. It's usually a small rubber bulb on the side of the generator, near the air filter. Press and release it several times (about 5-7 times). You should feel it firm up as fuel fills the line and carburetor bowl. Then, set the choke to full (if the engine is cold) or half (if it's warm) and try starting. It might take a few extra pulls or a longer crank than usual to pull the fuel through.
IF THAT DIDN'T WORK If priming doesn't do it, you might have a more stubborn air lock. Try this: turn the fuel valve OFF. Start the generator and let it run until it uses up all the fuel in the carburetor and dies. Then, turn the fuel valve back ON, prime it again, and restart. This can sometimes clear the vapor lock.
HEADS UP Avoid running it out of gas whenever possible. It's not just an inconvenience; the sudden loss of load can be hard on the inverter components. Always refuel before it's empty.