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.