Several years back while on vacation California had a rather interesting energy issue which lead to several outtages. One thing lead to another and the end result was me returning home to a crashed tank. This was my 100 gallon tank at the time which had various fish and corals.
I removed everything that was dead and which could be reasonably removed without more destruction. This was dead fish, snails, corals, etc. I then drained all water from the tank and sump, replaced broken powerheads and return pump, then filled back up with emergency tap water saltwater. Dechlorinated it, then let it do its thing. Basically kept a very short light cycle while I tried to figure out what to do next.
Not even 24 hours later the sandbed (which was rather deep @ 6 - 9" in various parts of the tank) showed life in it. I was pretty amazed. Rocks didn't show much. About a week later some zoa's that I didn't get cleared off all the way started to show some life. I did a water change about a week later and then put in a pair of clowns. I left it that way for about a year has I was pretty busy with other things and it was on auto pilot more or less.
Zoa's took over the whole tank. I later tore it down, moved the rocks and sandbed to two 29 gallons we set up, same with zoa's. The rock and zoa's seeded these tanks. The clowns I gave to a friend as they are too big for our tanks.
I guess what I'm saying is just clean up what you can or think is dead. Remove water, other things that are broke. Then treat it as a new tank. I think you may be surprised what lives. I know I was.