What would make my computer shutdown for no apparent reason, sometimes as often as once a day?

My computer has a habit of shutting down, I used to think it was connected with watching television on demand but it can also happen when I’m watching a DVD or even, as was the case yesterday, when I was on Fluther. It is a particularly difficult problem for me because I cannot use my hands and and restart the computer, if someone is here they can push the right buttons for me but if I am alone, which I am regularly, I am totally stuck. If you have any idea as to why this is happening I would be very grateful for your suggestions. By the way, it has been happening for at least 12 months.