Having a switched outlet and not knowing it is common and thus, a commonly asked question by many home owners, one that frequently comes up in older homes. Most home owners don’t even realize when one side of an outlet does not work when they purchase their home. It is common in some homes to have outlets that are partially controlled by a switch, meaning that either the top or bottom receptacle has the power source switched by a local switch in the room, leaving the other receptacle constantly powered (or hot). If you have this situation in your house and are looking to change it, this is a very simple situation to remedy.
The first and most common reason is that one side of the outlet is switched or at least it was at one point. It is not common for people to change a switch from controlling an outlet to overhead light. However, most people don’t think about the outlet that was controlled by that switch. Outlets that have one side controlled by a switch have had their bridge cut. The bridge is a small piece of brass that links both of the hot terminals and both the common terminals. The normal reasons why you cut the bridge is when you want one side of an outlet to either be on a different breaker, or controlled by a switch. Replace the outlet and nine times out of ten you have fixed your problem. When replacing an outlet make that has been switched, make sure you remove the second hot wire. Should you wire the outlet with two hot wires with the bridge uncut, this could cause damage to you or your home, and at the very minimum blow the circuit breaker. The second hot wire would normally get caped off at both the outlet and switch if that is the only thing between the switch and the outlet on that wire. Please if you are not comfortable with wiring consult a licensed electrician.
The second and less common problem I have seen that can cause one side of your outlet to not work is a simple bad outlet. Outlets have brass insides that conduct the electricity. Many of us have seen outlets that the plug falls out when you plug something in. That is because the brass has been bent and worn. Well in some cases the brass inside can break after years of use. Again the simple fix is to replace the outlet, re-wiring it exactly the same. This also applies if your plug falls out; replacing the outlet is the only fix for that as well.
Again, remember I AM NOT a licensed electrician and the information I have provided is only from my personal experience and knowledge I have gained while working around electricians. If you are not familiar with electricity and do not have the proper tools to complete your project safely and correctly, please contact a licensed electrician!