Take a peek at the rubber jumper from the sender to the line, back at the tank. The jumper is above the liquid line. If the line is cracked or the clamps loose, the pump will suck air there instead of bringing up the fuel. Actually, the pump doesn't suck at all, it just reduces the pressure in the tiny cavity within the pump. Atmospheric pressure enters your tank thru the vent, and PUSHES the fuel, towards that low-pressure area. If air enters the line somewhere else between the pump and the fuel, then the atmosphere just takes the shortcut.
BTW;
You will find a metal strap across that jumper; this is a ground for your fuel gauge. If you leave it off, your gauge won't work.
Won't crank, and won't start, or even won't fire, are all different things. And the cures for each of them are all different.