I thought I had it too, but I've noticed it only seems to happen to me when my daily free 15 fast-track images kick in. To combat it, I just close the tab, then reopen Bing Images, and that sometimes sorts it out.
Mine was OK after 30 minutes. Sometimes I'll save my prompt, then hold down 'ctrl' while pressing 'F5' which forces the browser page to reload. If my prompt disappears, I copy and paste it from where I saved it.