Sebaes are one of the most difficult anemones to keep, and need a lot of light. At least 8 watts per gallon. Unless you have strong halides or T5s, you probably don't have enough light for it.
The anemone being upside down and not attaching is a bad sign. Like others have said, try to put it right side up and get it to attach to something. If it won't attach, I'm afraid to say but it's probably long gone. If you suspect lighting is the problem, it may not be too late to give it to someone else with better lights to see if they can possibly revive it.