They will up-root themselves and flow with the current if they are unhappy. That might have happened with 2 days of no light. Put something light weight were the anemone was and let the current move it around the tank. Check there for the amenone, it is probably behind some rock. Also, ALWAYS check the sump when anemones disappear. I have salvaged 2 from the sump, one was pretty torn up by the powerhead and it made frags (no, I do not recommend fragging anemones this way!!).