The way this worked at the fab where I was, was that facilities would have paged everybody, and whoever needed to hold wafers would do so. You could mark your equipment down or unavailable for a particular step. I don’t know what we would have done if it was “hey, we lost dry nitrogen a minute ago.” I think at that point you lose a lot of wafers in wet cleans.
In the case of a power interruption at the fab, consequences were highly dependent on the equipment and the unit process. A prolonged power interruption to diffusion was the worst case scenario. You’d have 150 wafers in the furnace, and any significant deviation from the nominal temperature profile meant they were all scrap. Worse, if the furnace cooled off, you had to scrap the quartz boat the wafers rode in, too. Other processes had a smaller blast radius but were even more of a headache to disposition. Implant, you’d lose beam and probably lose vacuum too. Then the wafer in the chamber would be dusted and in an indeterminate state, and the rest of the wafers you’d have to sleuth out whether they were implanted or not. Sometimes you’d have a lot sitting in the end station and it wouldn’t be clear whether or not it had been run at all. At least in photolithography you could tell whether or not a wafer was patterned by looking at it.