There may be few reasons for that

1.Virus

2.Your Ram is going to get damaged, or is malfunctioning.

Install latest version of antivirus software(If you dont have retail version you can try with trial version), scan your computer.If it is due to virus and antivirus couldnt clean it, you may need to format your drive and reinstall the OS.

Check your Ram.Take it out and reinstall it.Sometimes this will be enough.If it is damaged you need to buy new.

If it is due to fan problem the computer usually shows warning before shutting down.

nepal ma mero computer ma BIOS ko problem le testo bhayeko theo

BE ko thesis report lekhni belama dheri choti testo bhayera maile sathi kaha gayera report banayeko

CPU lai thado parera rakhyo bhane time time ma off huni, ane kahele chai sutayo bhane off huni . k k... Ane CDROM kholyo bhane ta sure off huni ane kahele chai khuldai nakhulni, pachi eauta lai bechdiye maile teo computer, tesle kinera lagyo danga parera. sayad kineko 1 week pachi nai mother board change garyo hola, hehe.

there isn't sufficient information to diagnose this problem. There could be many things leading to shutting down a computer automatically.

Most of the times this is a symptom of a hardware problem.
Is it a laptop/ desktop?
Did you have any hardware upgrades?
How is your power supply?
Do you use batteries?
Do you have fluctuating voltages?
Is the CPU overheating?
How long does the computer stay on without being shut down automatically?
Does it shut down while specifically doing something, like reading a specific file or using an application?

Any messages/ warnings before being shutdown?
.............

What is the OS you're using? Since you're using MS Word, I assume you have XP/Vista. Check your power option settings from Control panel. Check if they are all correct.

What kind of CPU do you have? Computer make and model? RAM?

Well, if you are able to use internet, go to www.pcpitstop.com to diagnose the problems with your computer. This will help you to figure out any problems with your system most of the times.

To check if you have CPU over heating, which probably is, install a software that checks your CPU temperature. One of the tools I use is Everest.

http://www.softpedia.com/progDownload/Everest-Home-Edition-Download-16369.html

Download and unzip the file, and run everest.exe. This software gathers your system information. Go to Computer -> Sensor. Check the Temperatures.

http://www.pantherproducts.co.uk/Articles/CPU/CPU%20Temperatures.shtml

Check if you have temperature higher than the maximum temperature the CPU can withstand. If so, your computer is over heating. Check if the fan (on top of CPU) is working well, and heat sink is attached to the CPU.

Also check the motherboard tempearatures.

Does the computer have proper heat outlet? Keep the heat outlet away from walls.

Is it a laptop/ desktop

LAPTOP
Did you have any hardware upgrades?

DONT REMEMBER, ONCE I INCREASED MY RAM
How is your power supply? - SHOULD BE OKEY
Do you use batteries? - USUALLY POWER CORD
Do you have fluctuating voltages? - NO
Is the CPU overheating? - COULD BE
How long does the computer stay on without being shut down automatically?VARIES 1 HR TO 12 HRS
Does it shut down while specifically doing something, like reading a specific file or using an application?- NO PREFERENCES

Any messages/ warnings before being shutdown?- NOT AT ALL
.............

What is the OS you're using? Since you're using MS Word, I assume you have XP/Vista. Check your power option settings from Control panel. Check if they are all correct.

XP, HOW DO I CHECK THAT POWER OPTION

What kind of CPU do you have? Computer make and model? RAM?

ACER, INTEL PENTIUM,  1.7 GHZ, 1.24 GB RAM

Well, if you are able to use internet, go to www.pcpitstop.com to diagnose the problems with your computer. This will help you to figure out any problems with your system most of the times.

THNAKS

could be fan problem coz it makes a spinning sound for few seconds once i start and then kools down eventually

could be overheating coz the problem is more prominent when on flat surfaces

HI I am also having peoblem with automatically restarting. It shows some message but can't read it. Once it restart it says "your computer has been recovered from the biggest threat". It happens sometimes 5-6 times in one day. Also the computer is very slow.

BTW , I am using Dell Latitude D610.

Thanks for any recommendation!

blue moon you can resolve your problem

go to my computer -properties-advanced-startup and recovery settings-under system failure uncheck autopatically restart

sunima_sh,
did you check the CPU/motherboard temperatures? Install the EVEREST tool as I mentioned above. Check if it exceeds the maximum CPU temperature that your CPU can withstand. Check one my posts above for more details. Make sure your heat outlet is not blocked.

Which model of Acer do you have? You can get the details from : Control Panel -> System -> General tab (XP classic view); Control Panel -> System and maintenance -> System -> General tab (Visa & XP category view).

blue_moon,
unchecking the 'automatically restart' will NOT solve your problem. This option is there when your system fails, otherwise it will simply shutdown. sunima_sh, do you have this option unchecked? It may be the reason that your system is simply shutting down without a restart due to system failure!! I do not recommend unchecking this option.

Most probably you have 'Spyware' problem, as most of the people these days have. This is the most common problem these days. To check if you have a spyware/adware, go to:
www.pcpitstop.com
click on full tests -> new members -> create an account (you dont have to) -> Let's go -> OK
When the results are displayed, click on "Test Details" on right under "Current Test Results". In the "Running Programs", check if any program is listed as spyware/adware.  You need to remove them. They collect your personal information like shopping habits, what you search on internet, activities. They run on your computer all the time, hence making your system slow.