Tanks come in a variety of sizes. The standard tank that we ship out with our systems weighs about 9 pounds when empty. If your tank has the same dimensions, its "empty weight" should be about 9 pounds. To see if your tank weighs 9 pounds when empty, make sure the valve on top of the tank is open. Then open your faucet and empty your tank completely. Then put it on a scale. If your tank weighs significantly more than 9 pounds, the bladder inside the tank probably has a leak in it, and the water inside the tank may be contaminated. Close the valve on top of the tank, and purchase a new tank. In the meantime, you can still use your system by placing a glass or jug in the sink, opening your faucet, and allowing your system to slowly fill it up.
Another possibility is that your tank has lost its air pressure. The side of the tank has a air nozzle (like on a tire). If you put an air pressure gauge onto it, it should register 5-7 psi. If it is less, just pump a little air into it.