To give you some perspective, Travis, I just bought a piece of rock from my LFS
. It came in as "Assorted mushroom on rock" with 4 mushrooms ranging from 1-2 inches in diameter. But they had a light failure and corporate policy said they had to wait for an approved repair guy to come fix it. They were stuck with an unlit tank for two weeks!
The mushrooms got dull and shrank down to about pea sized. So they sold it to me as rock. In a week, the mushrooms have doubled in size again. They survived two weeks with no light.
Since mushrooms are corallimorphs (not true corals, not really anemones either) they share properties with both coral and anemones. Many anemones, when starving, are able to self digest to wait out a famine or period of no light. So, if your mushroom is still in one piece, odds are that it will recover, but it may take some time. Marquis was spot on. It's hard to kill mushrooms unless you're setting out to do it intentionally!