Commonly used by writers on role-playing forums, a "muse fountain" is a slang term for your inspiration for certain plots and characters, or perhaps your muse as a whole. A dry muse fountain, therefore, implies that you have ran out of inspiration for whatever you are referring to.
Example 1: Ugh. All the stuff that I'm dealing with has totally dried up my muse fountain! I don't think I can finish any more of the book tonight.
Example 2: My muse fountain is overflowing! I haven't had this much inspiration in a while!