Some of these causes could include one or more of the following:
1. If your engine has one, a non-working electricly operated fuel cutoff solenoid valve [used to prevent "run on" after key is turned off]. In operation, a spring closes this valve, cutting off fuel flow into the carburetor, and when the key is on, the coil in the solenoid operated valve opens it. If the coil is failed, or the wire supplying voltage to the valve is "open," then the valve STAYS CLOSED, preventing any fuel flow. Check first for a disconnected, or cut or broken, wire to the solenoid valve.
2. A blocked fuel filter or filters.
3. A non-working fuel pump.
4. A stuck [in the closed position] carburetor float valve.
5. Blocked passages [galleries] in the carburetor.
6. Blocked jets in the carburetor.
Numbers 4, 5, and 6 happen very seldom. Hope this helps, good luck. j3h