Your computer can become fried due to one or more of several reasons. Commons causes of a computer being fried include an electrical surge in the computer's hardware and excessive heat generated by that hardware. Electrical surges can occur due to the power grid and electrical storms. Damaging heat can occur because of an improperly ventilated case, overclocking or even prolonged and hard use. Whatever the cause, you can attempt to fix your fried computer before resorting to paying a professional to do so.
Remove all cable connections from the back of your fried computer. Press the power button to make certain no charge remains in the computer. Place the fried computer on its side on a flat and clean work surface.
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Remove the side panel of the computer so that you can see all of the computer's internal components.
Search for obvious signs of damaged components such as black scorch marks and swollen capacitors.
Remove any obviously damaged components.
Remove each individual component one at a time. Place a new or working component in its place. Plug the computer's power cable into its power supply and an available power socket. Attempt to turn the computer on after inserting a new or working component into an old component's place. Replace each new component with the old component after each test to keep the parameters of the test the same for each component. This is a process of elimination. If the computer turns on successfully, then you have located the fried component or components.
Replace all of the old components with new or working test components. Turn the computer on. This tests the motherboard itself. Should the computer still not turn on, it is possible that the motherboard is at fault. Test the computer with a different power supply at this point as well, because if the power supply is fried then no power will reach the other components. If you discover that the power supply is also burned out, after replacing it with a new one reinsert your old components and turn the computer on after each component to determine if any of those are also burned out.