You don't say what vehicle you have. Generically speaking I would proceed as follows:
1: Pull the line from the pump to the carb at the carb and turn the engine over. Are you getting gas? If so it's could be the needle valve in the carb sticking closed. If not work backward to the line between the pump & carb.
2: Pull the line & blow through it. Is it plugged? If not work backward to the pump.
3: Pull the pump and manually work the lever with your fingers over the intake & output ports. Is it pulling a vacuum at the intake port and creating pressure at the outlet? If so work backward to the fuel line leading into the pump from the tank. If not the pump is bad.
4: Pull the line between the tank & pump. Blow through it. Is it clear? If so work back to the connection between the tank & line. My 38 had a petcock there that was plugged.

If you are clear to this point it about has to be something in the tank that's restricting flow. Good luck.

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