If your powerplant gets damaged beyond a certain point, its output will (temporarily) drop to 40% of its rated power - shutting off all modules that can't be supplied any more. In case of my
Liquid Spirit (evac Phantom), priorization won't help any more at this point, since my thrusters alone use up 48% of the maximum power... in a combat build, you might be able to priorize your modules so that at least the thrusters will stay online, even if everything else gets rebooted.
If you take my combat mamba for example
A ship builder, outfitting and comparison tool for Elite Dangerous
s.orbis.zone
you'll see that, at 40% power, I can still run thrusters and FSD (now that we're talking about it - I might even shift the shields into prio 1), so I could still run, even if everything else is rebooted.