Here's a link all about caring for dedrobium nobile orchids. After reading it, you should be able to identify the problem. Good luck.

http://www.orchidsmadeeasy.com/dendrobium-orchid-care/

I suspect it has moved from one temperature to another and it doesn't like it. avoid cold draughts or radiators. Is it sitting in water if it is drain it and leave it to dry out abit.