The tank is waterlogged. The air bladder has no pressure.
Fix: Shut off the feed water and open the faucet to drain the tank completely. Check the tank's air pressure with a tire gauge on the Schrader valve (like a bike tire). It should read 7-8 psi with zero water in the tank. If it's at 0 psi, use a bicycle pump to inflate it to 7-8 psi.
If that didn't work: The bladder inside the tank is ruptured. The tank will not hold proper air pressure and needs to be replaced.
Heads up: Normal tank delivery pressure is about 30-40 psi when full. If your faucet flow is weak but the tank is pressurized, check the tank ball valve and the faucet aerator for clogs.