as i switched my generator supply to the supply provided by the government with the help of change over (selector) the computer restarts and the PSU didn't start up and so did the computer
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How the computer can start up while the psu does not.
I think when you put it to utility line 220v only Monitor/Lcd turned on and main/mother board did not.because there is no relation between the psu and Monitor/Lcd.They both have separate power supplies.
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So the voltage in our country is 230v.Almost all computers no matter brand new or used are sold with the switch set to 230v.So until you switch them to 120v,if so then don't even think about to repair it.It is a switching power supply.After all not the job for unexperienced people, you could get killed or injured if you touch high voltage parts inside.
Rectified DC voltage of 300v is much more dangerous than AC 230.
when it happened did you listen any bang or chirping sound or smoke or burning smell?
SMPS could die without any smoke or smell or bang.
There is always a fuse inside them,may have blown.
If the switch was set to 120v and PSu put to 220v, The fuse may have blown along with main filtering capacitor exploded or at least badly heated up and damaged.
PWM chip or Other PWM circuitry damaged.
And if not any of above conditions and switch is properly put to 230v setting then a sudden surge( mains voltage increased )could have killed the psu silently.And you could not know it (That happens sometimes in our country. Last year the voltage in my feeder line of "WAPDA" suddenly went to 440v And every appliance running in my home was damaged beyond repair).
Another reason could be just aging. Inside the psu are a number of Capacitors at the outputs of it.They can die with aging and Main board will not get a filtered voltages and may not start up or shut down shortly after startup.
This problem can be pointed out visually, by opening the PSU and without putting it to power.The top side of effected capacitor may look bulged or some leaked liquid electrolyte under the capacitor is sign of damaged capacitor.It happens to almost all cheap PSU.But the capacitor may not show any of above sign and still dead.
In every case mentioned above, you may not or should not try to open the PSU to repair it.Unless you have at least moderate level of experience in repairing mains operated circuits and have tools required to do so.
That should be enough for this topic(although it could be explained much longer) but no need to do so.
Long story short, throw it away and buy a new one good quality PSU.The repair cost will be much higher than the price of a good condition little used PSU.