My pressure tank is 4yrs old. It had been filling up and shutting off fine until last night. I noticed that the well pump was running but no water was entering (filling) the pressure tank? Water would bypass the tank and feed the pipes in the house with good pressure. What could it be?

Tank bladder could be bad and tank is waterlogged.

If you do not replace the bad air tank you will be replacing the pump within a day or so.