Power surge or lightning strike.
Power Supply Problems
Computer power supplies are often taken for granted even
though the job they do is complex. While every power supply takes the
electrical current from an AC outlet and converts it into something that an
appliance can use, a computer power supply must do so at several different
voltages at once.
A faulty power supply can cause many random computer issues
and learning to diagnose them can save you a great deal of time.
Recognizing an Overloaded Power Supply
Most power supplies have variable-speed fans.
If the fan always emits a loud, whining noise, there might
be a problem.
This can be one of your first clues that you might be
stressing your power supply.
First, check the power supply itself.
Some power supplies have two cooling fans and one may need
to be connected to the computer's motherboard.
If there is only one cooling fan, examine it. Is it dirty?
The fan can be cleaned with a cotton swab dampened with a
little rubbing alcohol.
If the power supply is clean and the auxiliary cooling fan
(if applicable) is connected, you may be working the power supply too hard.
If you have a powerful computer with several hard drives,
multiple video cards, and a speedy processor, temporarily disconnect a few of
these components to see if this cures the problem. If it does, you will need to
consider purchasing a new power supply.
An overloaded power supply may cause your computer to lock
up or reboot when the computer is working particularly hard, such as when you
are playing a game, rendering a complicated graphic image, or recording a song
with real-time digital signal processing.
If you are experiencing these symptoms, consider replacing
the power supply.
Recognizing a Failing Power Supply
A failing power supply can be difficult to diagnose. An
audible signal may be your first clue, though. If you hear a grinding, buzzing,
hissing, or clicking sound from your power supply, replace it immediately. It
may be a serious fire risk.
Not every power supply fails in such spectacular fashion,
however.
If your computer shuts down during the boot process or
randomly while you are using the PC or if it locks up or reboots when you are
not performing a complicated task, the power supply may be to blame.
Safety Note
Computer power supplies contain capacitors that hold a
potentially dangerous charge.
If you are not completely comfortable around electronics, do
not attempt to clean a dirty power supply on your own.
Have an expert do it for you, or discard the power supply
and purchase a new one.
http://www.fonerbooks.com/power.htm
http://www.tomshardware.com/reviews/stress-test,1073.html
http://pcguide.com/ref/power/sup/index.htm
http://www.emachines.com/
