Chances are it is either losing it's spark or it's fuel and there is a small chance it is suffering from tappet jacking - a usually temporary condition where an abnormally high oil pressure forces the hydraulic tappets to extend jacking the valves from their seats causing a temporary loss of compression. When trying to restart, such an engine spins easily as if there is little or no compression but extensive cranking usually restores things enough for it to run, firing first on one cylinder and then in time the others join in.

If that doesn't sound like your engine, it is probably time to check the fuel delivery volume and pressure and cross that off your checklist...