If the consecrated murti in a temple gets damaged or destroyed, what is the scripture-prescribed way to reconsecrate and reinstate the deity? Until such reconsecration is done, how should the temple be maintained since regular worship wouldn't be possible?
A partial answer
In Mahanirvana Tantram Lord Shiva says idols, which are broken or which are otherwise defective, must be immersed in water. Such idols should never be worshipped. New idols should be brought and duly energized as per the prescribed rules. And then the worship can be resumed.
Shri Sadashiva said:
If there be an omission to worship an image for a day, then (the next day) the worship should be twice performed; if for two days, then the worship should be four times performed; if for three days, then it should be celebrated eight times (97).
If the omission extends three days, but does not exceed six months, then the wise man should worship after bathing the Deva with eight jars of water (98).
If the period of omission exceeds six months, then the excellent worshipper should carefully consecrate the Deva according to the rules already laid down, and then worship Him (99) The wise man should not worship the image of a Deva which is broken or is holed, or which has lost a limb, or has been touched by a leper, or has fallen on unholy ground (100).
The image of a Deva with missing limbs, or which is broken or has holes in it, should be consigned to water. If the image has been made impure by touch, it should be consecrated, and then worshipped (101).
The Mahapithas and Anadi-lingas are free from all deficiencies, and these should always be worshipped for the attainment of happiness by each worshipper as he pleases (102).
Mahamaya! whatever Thou hast asked for the good of men who act with a view to the fruits of action, I have answered all this in detail (103)
Mahanirvana Tantra, Chapter 13
And, the Prana Prathishtha (energizing the idol with life-force of the deity) method varies from deity to deity.