Just the obvious first...Make sure you have gas in the tank. Then:
1. Try to start it again. (You'd be surprised)
2. Check the low tension wire to the distributor to make sure it did not fall off. Same with the coil.
3. Pull the cap, turn on the ignition, and crank the engine while looking at the points in a fairly dark or dimly lit area and make sure you have spark at the points. While you are under the cap, make sure the rotor is properly seated, the condensor screw is tight and the cap looks ok. If you find problems here, tidy things up, put the cap back on and start the car.
4. If you have spark at the points, but it still won't start pull #1 plug, reattach the plug wire to it and just set the plug electrode on the head or block for gound and then crank it over and see if you have a spark at the plugs. If no spark to the plugs, you have a problem between the rotor and the cap. Maybe even wet plug wire connections. But all of that is unlikely.
Assuming you have spark to the plugs and it still won't start...
5. Next, pull the fuel line AFTER the fuel filter, preferably right where the fuel line goes into the front carb. Stick the fuel line into some kind of reasonably safe container and turn on electric pump or crank engine to work the mechanical pump and see if you are getting fuel to the carbs.
6. If you are not getting fuel, check to make sure your fuel filter is not clogged. If it is not clogged, next You can disconnect the fuel line going into the fuel pump. Fuel should come out of the disconnected line if the tank is full. On a low tank, this may not flow. I've seen guys suck on the line... I don't advise that. But you get the idea here. Make sure you have a flow of fuel to the pump. If you do, and the pump is not moving gas to the carbs, you wither have a bad mechanical pump or a problen with an electric pump that you may be able to fix without replacing the pump.
7. If you are getting fuel to the carb, but it still isn't starting, then pull the cover off the bowl of the front carb and unstick the stuck-closed float valve. Fuel will now enter the carb bowls and the car will start. Make it a point to replace the float valve needle and seat to prevent it from happening again.
In the past, I have had the stuck float valve and have curred it for the moment by hitting the top of the float cover with a screw driver handle. This shakes the needle open in some cases and you don't have to do any of the above steps, just replace the needle and seat. But a really well stuck needle will not shake loose.
Good luck.