Sorry to hear about your plate coral. Have you read through the info available on wetwebmedia?
http://www.wetwebmedia.com/fungiidae.htm
Might give you more insight as to why your plate is doing this.
Anyway, it has been mentioned that even though a plate coral might look dead (no flesh just skeleton) there have been cases when they've made remarkable comebacks by spawning off baby plates. Check this out:
http://www.wetwebmedia.com/fungiidreprofaqs.htm
Has the plate been stung by anything close by? I had a ricordea mushroom fall onto my plate coral and it stung it to the bare skeleton.
Good luck.