CMOS Battery
Lifetime event: I replaced the motherboard cmos battery in an ancient PC (2010) after it became troublesome and in the end wouldn't boot. That's a lifetime first. The fact that the bios lost date and time a week or so back was the clue. Apparently the advice is to replace them every 5 years but I am not made of money.
However, in addition to now booting, the whole machine seems sharper and more responsive somehow. So to those with old machines perhaps change the battery ?