What EXACTLY are the steps to take to prevent further unexpected shutdowns?

More details would be great before we can even think about what the underlying issue might actually be.

I have a Dell notebook which shuts down a lot. I vacuum the side where the fan is. But it does get far too warm on the power supply/fan corner so I have to wait for it to cool down.

I make sure there is air under it too. I have it propped on something but you can buy little stands and even little USB fans that go underneath.

If yours is a desktop, take the cover off the CPU and vacuum around inside. You will find tons of dust.

Try a cooling mat. If you upgraded you OS the added strain may be causing the over heating. Slow fan... There are lot's of possible causes