Cindy. Welcome to the forums.
Cindy, what everyone here is telling you is that we need a lot more information.
First of all, there is
a reason it is shutting down. We just don't know what that reason is.
Here is a list of things that may help us:
Laptop or desktop system?
If desktop system:
Configuration (what parts are in it - motherboard, memory, PSU, video card, case)?
Is it stock, overclocked, or does it contain special hardware??
When did the problem start?
What are you doing when the problem occurs?
What software is running when the problem occurs?
Is the problem repeatable?