The tank should have 2 pounds less than the turn on pressure setting. Shut off the pump, open a faucet, read the pressure. If it is zero and you put 40 pounds in it recently, it is leaking out and the bladder is probably shot. Try to pump the air pressure up while the faucet is opened with the pump off. If the bladder is bad, as you add air, you'll push water out of the tank, but you will not be able to build up and maintain thatpressure. Leave things off for maybe 1/2 hour or so after (if) you get the pressure raised with a faucet opened. If it drops, again, either the air valve or the bladder is leaking. You could try some soapy water around the air valve and see if you get bubbles, but my guess is that the tank is probably shot. The telltale symptom of no air in the tank is if it sounds like a dull thud as you rap it from top to bottom. The part where the air is should ring sort of like a bell, and sound quite different from the part that has water in it. Especially after just getting filled up, the part with water in it will be cold, while that with air in it will be warmer. If after turning the pump back on get get some water, the pump works. Now, it could be that the well is not producing as much water as it used to, and that is a problem, too. Worry about that once you figure out if your tank can maintain pressure. Note that the empty pressure will rise to the upper shut off pressure from the pump after it stops (with no faucet opened.
There is also a check valve somewhere between the tank and the pump. This only lets water go from the pump to the tank. If the valve is bad, the pressure tank will push the water back down through the check valve (one-way valve) eventually causing the pump to turn back on to maintain pressure.
So, there could be lots of things wrong, but often it is the tank. You need to elimiinate them one at a time. The tank must be able to hold air pressure, and that is easy to check so start there.