My computer keeps shutting down

[mrscross]

My HP computer keeps shutting down. Initially it was when we were using Minecraft so I assumed it was a graphics card issue. But now it is shutting down mid-way through any internet use and even typing into a Word document.

[Hibb1]

So when you say shutting down, so you mean Windows actually says it's shutting down? Or does the machine just switch off instantly?

[mrscross]

It loses power and switches off without any warning. It’s as if the power has turned off. It doesn’t restart straight away. We have to leave it for a period of time (that seems to vary) and eventually we press the power on button and it turns back on. 
It can happen minutes after we turn it on or not at all for hours of use. There doesn’t seem to be a pattern to it other than it is getting more frequent. It doesn’t seem like a heat issue or a software issue.