Sorry to hear the bad situation ...
Do not flip the switch at back of Power Supply if thats what you did. Its the voltage setting and if you increase it, thats what happens ... the Power Supply burns out.
Anyway, I believe, the motherboard may be bad or could be something wrong with memory modules.
Follow these instructions please ...
Unplug all the cables from your computer.
Plug-in ONLY monitor and the main power cable to the tower.
Try to start it and see if you can see anything coming up on the screen.
Make sure the monitor is working fine. If you have a different monitor to try, try to use it with your computer.
If you are sure the monitor is working fine, lets try something else.
Unplug all the cables from tower/computer.
With the cables unplugged, please press and hold down the power button at front of tower for atleast 10 seconds and then release it.
Try to start your computer now with power and monitor cable plugged in.
If nogo, unplug all the cables and open the computer case.
Remove all the PCI cards (graphic card, sound card, dial-up card, etc.).
Try to start computer now and try to feel if there is any change.
If nogo, unplug all the cables that are connected to CD ROM and Hard Drive. These are flat belt-like cables going from mother board to Drives.
There is a small coin size battery. Remove it. To remove it, there is a small silver clip which holds it down, squeeze it with the round plastic and the battery should pop-out.
Keep the battery outside for around 30 secs and put it back inside, please make sure you put the battery exactly the same way you took it out. Most of the times, + sign on battery is facing up.
Try to start the computer and feel if there is any change.
If nogo, shutdown computer and press and hold power button for atleast 10 secs to release the flea-power.
Now, remove all the Memory modules. Push the clips on each end away from RAMs to take them out.
If your system has two RAMs, take both of them out, insert one RAM at a time in both slots and try to start the computer.
Try RAM1 in slot 1 and repeat starting process, Try RAM1 in Slot 2 and do same again. Similarly, try RAM2.
If nogo, we may have to reseat the processor.
If you can figure out where is the processor, you will see a big square metallic piece on top. Thats the heat sinc which takes all the heat from processor and throws it out of your computer.
There should hinges/clips on sides of the heatsinc. Remove the hinges and take the heat sinc out gently.
BE CAREFUL while taking it out and make sure it is not taking the processor out with it.
Once the heat sinc is removed, put it upside down on a safe place. Do not remove the flux (which may be dry by now) from the base of heat sinc or from the top of processor.
The processor may be inside a cage or you may find a small lever which can release the processor.
Take the processor out using your nails. DONT use anything metallic against it. Do not force to remove it out. Be gentle with this part. Once removed, keep it aside with the pins facing up. Check if you can see any pins bent or broken or if you can find any burn-marks on the processor.
If everything looks alright, put it back. While putting it back, please check, there should be 1 pin missing on the processor and one hole on the base would be blocked. We need to match the missing pin and the blocked hole and then gently push the processor back into its position.
Put the heat sinc back on top of the processor. Lock the hinges.
Put the RAM back into its slots.
Try starting the computer and see if you can feel any change in the way computer starts.
You may plugin the monitor cable at this point.
If the computer starts up, VOILA!!
If the computer still acting up the same way, it could be quite possible one or more major components affected.
It could be Motherboard or Processor causing this situation.
Buy the motherboard. Replace the motherboard with a new one. If that works, great. Otherwise, return the motherboard and get your money back and buy a similar processor.
I hope this helps ...
Good Luck.