I wrote a post about our new pressure tank, which has an air bladder installed in it. Our old pressure tank does not have a air bladder installed. Pressure tanks work under the principle that gases can be compressed, while liquids cannot. When the pressure in the tank drops, the well pump turns on and pumps water into the tank, compressing the air bubble at the top of the tank. If there is no air bubble, the pump will cycle on and off very quickly causing a possible motor burnout in the well pump.
If you have a bladder tank, the installer should have charged it properly, and once charged, it will not loose its air charge. A non-bladder tank will, over time, loose its air charge because the air will dissolve in the water and disappear out of the various faucets. Therefore a non-bladder tank should be recharged regularly with air. If you suspect a tank is completely water logged, it will not hurt to put a little air in and see if it makes a difference. The air fittings look like tire fill valves, and are normally located near the top of the tank, possibly next to the pressure gauge. You can add air with a bicycle pump, I would start by adding enough air to make the pressure gauge go up 10 PSI.
To completely recharge the tank, turn off the pump and run the water until the pressure gauge reads zero. Add air until you reach the pump cut in pressure (low pressure setting where the pump turns on). Then turn the pump back on. This is the proper amount of air for your pressure tank. The pressure gauge should go up slowly until it reaches the cut out pressure (high pressure setting when the pump turns off).