Water pressure slowly decreased tonight right down to nothing. Square D Welltroll Pressure switch contacts look OK and both sides have 110v on meter. It is the 40/60psi switch. Disconnect switch for pump also looks OK and has 110v on both sides. Pressure on water line after pressure switch is 90 psi (think it is bad huh). Air valve shows 0 psi and very little air came out when I pushed valve stem in. Any help where to look next would be appreciated. I am thinking of adding 38psi of air to tank and see what happens, but shouldn't pressure switch turn on pump even if tank is at 0psi air?

Adding air to your tank won't help at all. It sounds like your pump is not coming on. (I am assuming the pump is in the well.) The contact points on the pump should be closed and you should get 220 volts when you use a voltmeter to bridge the two leads going TO the pump. If you are getting current to the pump, then the switch is doing its job and you start suspecting pump problems. Does your pump have a capacitor box above ground? That would be a electrical box between the switch and the wire going into the wellhead.

You can, by the way, add 38# of air to the tank with the pump off and pressure at zero, but it is quite likely that, with zero pressure, your bladder in the tank is ruptured. You should still have pressure at the top of the tank if the bladder is good. With pressure at zero, it sounds like, as I said, the bladder is bad. If that's the case, then your pump might have been short cycling for several weeks/months. That could cause the pump to go bad.

If the pump pumps up to the cut out point and then shuts down and loses pressure then it sounds like a faulty foot/check valve.. Change the valve and pressure the bladder up to 38 PSI as suggested and see if that doesn't solve your problem. Good luck, Tom