how can I make my stuck Mp3 player work again?

I have Zen Creative,Style 100..I gave it to a friend to put more music on..he changed t configuration in library and told me,he've put it into a folders..I turned it on this morning to listen to it and all seemed fine,but when I turned Now playing off,it just got stuck on the main menu( on Music folder) with a screen in original colour and all that but with black stripe on it...help please,I don't want to loose t music!!! Cheers!