i installed a switch on my water pump and nothing is happening?

With some presssure switches you have to manually flip the lever on the side of the switch until the pressure rises above the "on" pressure setting. After the initial start it should work fine.

If you have power leaving the pressure switch going to the controller, you have a whole new set of possibilities.

Did you install a pressure switch or an on/off switch? If pressure switch you need to check output from the switch to narrow it down a bit. Some pumps have an overload switch that may be tripped or defective, a start relay in the control box could be bad, the hose from the tank to switch or the tap in the pump body could be blocked with rust or debris, or the pressure switch could be stuck or improperly adjusted, or the pump may be defective. If it was an on/off switch, where in the circuit was it installed?

Is it adjusted to the correct pressure settings for on and off and the correct differential pressure? These are the reasons that a pro should install and adjust pressure switches.