computer shutoff suddenly, I have a APC backup UPS SE 350

"With the APC ES 350 (there's no SE model) there's two audible alarms:

A continuous low beep: The UPS is now operating in battery mode. This happens if you experience a power outage and the UPS switches to the internal battery. Depending if you are operating at 100 or 200 Watts, you have between 1 and 6 minutes of battery time.

A continuous sound: (and not repetitive beeps) informs you of a power overload. You got a power spike and the UPS switched on to battery mode. The UPS will not resume from this mode until you turn it off and turn it back on. If the spike persists it will again repeat the tone.

The fact your computer turned off immediately means your battery was discharged when this happened. This model takes around 16 hours to recharge its internal battery. If it is continuously connected to a power outlet even when the computer is turned off, this may mean your UPS internal battery is not in good condition.

To test your battery condition:

Boot your Windows 7 up to the logon
screen but don't log on to avoid file
corruption.
Remove the UPS power cable from the
wall socket.
The UPS should start a low beeping
sound and your computer should stay
on for 1 minute (if your UPS is
operating at 100 watts), or 6
minutes (if at 200 watts).

For more information:
APC UPS ES 350 Details
Also check your UPS manual for even more detailed information."

"There's two basic reasons for a UPS to fail to keep a PC running through a short power outage...

Too much load - Remove your monitor and other non-critical items from the ""battery backup"" ports and compare your PC's power supply wattage against the UPS output. For a consumer UPS the model number (350 in your case) often indicates the absolute maximum wattage it can supply. For best results it should be higher than the wattage of your PC power supply.
Bad battery - UPS batteries last about 18-24 months. If your UPS is older then it likely needs a new battery. A replacement battery will cost roughly 40% the cost of a new UPS. Batteries Plus is a good place to find a replacement.

** Numbers above are estimates from my experience and should not be taken literally."