There was a blip in the electriciy. That shouldn't cause me a problem, everything has UPSes. But after the blip, two of the UPSes were off, so lots of hassle for me.
One of the UPSes was the one upstairs. That's not a problem, it's just workstations. I restarted the UPS and it was fine, I could log into things.
Or rather, I couldn't.
Investigation revealed that the UPS in the cupboard under the stairs had failed too. And that powers the DSLs (unimportant, except that ladysolly's iPad couldn't reach the internet) and the 32-way distribution switch, which is very very important, because it links pretty much everything together.
So, I replaced that UPS with one that wasn't doing much, and now that's looking good.
Then I discovered that another UPS had failed. It was powered-on, but it had lost power for a short while. As a result, five computers had rebooted, and a lot of what needs to be run at boot-up, isn't automated (that's deliberate). So I took that UPS out of action, and connected those five to one of my "BIG FIVE" UPSes, one that wasn't carrying much load.
The root of the problem is that the batteries in UPSes are sealed lead acid (SLA). SLA batteries don't last for ever, I'd estimate that they'll be good for three to five years. And the failing UPSes were last re-batteried in 2010.
I took the one from the DSL cupboard apart, and re-batteried it with a set I've only just bought. But when I tried to power it on, it just sat there sullenly, refusing to wake. So that's probably no good now. I also took the one that had been powering those five computers apart, found that one of the batteries inside was duff, and replaced that with a goodish one (I say goodish, because although it's a few years old, it was showing 12 volts). That is now my only spare UPS, although I have another that's covering three computers, and those could go onto one of my BIG FIVE.