As you read this article, please bear in mind that this is just a generic guide and not meant to be a complete set of instructions. You are dealing with an electric motor. Electricity is extremely dangerous. If you are uncertain about any of this, do not attempt to repair this on your own; contact a suitable service technician.
At some point over the life of your pool pump you may turn it on one day and all that happens is you hear is a humming sound. If this happens, quickly turn off the pump. There are several possible causes. The most common time for this to occur is after the pump has been sitting idle for a long period of time (such has over the winter). Scale or other corrosion can build up in certain parts of the motor and make it difficult to start. With the power turned off, examine the back of your pool pump's motor. You will need to remove the cover and manually spin the motor shaft. If the shaft spins freely, replace the cover and try the motor again. If not, you will need to disassemble the pump to determine if anything is clogging or jamming the impeller.
If nothing is jamming the impeller and that shaft still will not turn, you will probably need to replace the motor. If the shaft spins freely by hand, but still only hums when you turn on the power, you will need to use a multi-meter to verify the pump is receiving the correct amount of electricity. (remember, if you are not properly trained on how to safely perform any of these procedures, hire a service technician or an electrician). If the shaft spins freely by hand, you have the proper amount of electricity, and the unit still only hums, you will probably need to replace the motor.
In many cases, simply spinning the shaft manually to break it free then turning the power back on the correct the problem.