Diagnosing a pump problem requires understanding two aspects of the system.
1. How does the electrical power get to the pump, and
2. How does the pump put water into the system and deliver pressure to your plumbing.
Is the pressure sensor a gauge or an electronic digital device?
Can you get any water if you open a valve near the gauge that you can see? You want to find if there is pressure in the tank represented by the 45 psi reading, or if that is wrong.
If there is real water pressure in the tank, then you should be able to get water from some point nearby.
A homeowner should own an electrical meter (usually called a multimeter) and learn how to use it. They start at about 10 to 20 dollars at Sears or Radio Shack, more at HD for a better meter. You will need it to see if you have power to the pump. Another type is a clamp-on ammeter which is useful for those with pumps, but costs more (about $80 for the least expensive that I have seen at HD) and is probably not an investment that most homoewners would make.
Is there a pressure switch with a cover that you can remove? If a pressure switch, then that controls the pump. If you remove the cover there should be some visible contacts. They are electrically hot, unless you turn off the breaker to the pump.
The contacts should be closed if there is no pressure, and open if the pressure has turned off the pump.
There should be wires coming from the breaker, and wires going to the pump (or to a control box if you have a 3-wire submersible pump), with the pressure switch contacts in between.
You can usually see if the contacts are closed or open.
If you have a meter, then with the circuit breaker on, measure the voltage between each of the incoming wires and between each of the outgoing wires. If no incoming power, check the circuit breaker. If no outgoing power and the contacts are closed, then you may have a switch problem. If there is outgoing power but no pumping, then you may have a motor problem or a control box problem if you have a control box.
Is there a button on the control box (if you have one) that you can push that is maybe a "reset" button?
Possibilities are too many to carry this on. Check out what you can and report back for more suggestions.