Right Ship Panel -> Functions -> Option "Reboot/Repair"
If it doesn't do the trick on 1st attempt, repeat process until the Ship becomes mobile again
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(which can happen, if multiple Modules are at 0 (dead) or below 5% health)
If the Power Plant is severely Damaged, it'll only yield 50% total Power Outpot.
That condition requires Power Management.
-> set Thrusters, Life Support, Sensors and FSD to Highest Priority (1), while all unessential Equipment for flying the Ship are set to (2) or lower Priority (5 being the lowest)
--> this allows to fly the Ship back to the nearest place for repairs (beware that not enough Power might be available for Shields, so the repair-trip possibly has to be made without them active)
--> naturally, keep Hardpoints retracted... Ship won't have the Power for those anyway under most circumstances