Why would a laptop turn off after 15 seconds?

I have a laptop that my late father bought for us. Today, after owning it for about 6-7 years, it won't turn on. Or rather, it turns on, and after 15 seconds, it turns off. The screen never comes on in that time. Even though I worked in IT for 5 years (a while ago, admittedly), I've never seen this, nor learned any skills on even beginning to diagnose. Any and all tips most welcome. It may be old, but in terms of spec, even something equivalent , today, would be way more than we can afford at the moment (no sob story, I promise), and would disrupt our life significantly so I want to put due di

Question by Dr. Kaii on Super User, licensed CC BY-SA. We show a short excerpt and link to the original thread as credit.