Writer's block! Burnout! We all go through it and each of us have our ways of dealing with it. Here a list of mine, and why and how they work:
1) Keep more than one writing project going. If my Muse burns out on one, I can go to another. It keeps my projects from stagnating, my Muse from crapping out on me, and helps me to still get stuff done. I have at present three WIPs going on at the same time.
2) Write for yourself, don't write for deadlines. If all you want to do is write a crack!fic about your favorite fandom or a silly little piece that you'd never want to publish in a million years but you just have an ITCH to do it...just sit down and DO IT. As long as you're writing something, you're staying fresh--and you may get inspiration from doing it that will help with your current works.
3) Walk away from the keyboard!! It's okay to take a break. In fact, it's good for us and our writing. We may come back to find things we didn't see before in our writing that needs work and therefore have a fresher perspective on the material.
4) When you're completely stuck, just do the things you love. I play World of Warcraft online, watch episodes of Torchwood, read Doctor Who fanfic--whatever allows me to relax and give my chance for my Muse to get new material. Sometimes even watching movies in similar genres that I'm writing in allows me to be able to get inspiration I desperately needed.
5) If you can, take a vacation--especially if it's related to what you're writing about. I have on limited times and budgets gone on a holiday that lasted all of one and a half days with a single night overstay. The purpose was to gauge the area--namely, the West Village in NYC--for the backdrop of my novel, Blood of the Dark Moon. It not only turned out to be very educational for my book, but gave me a LOT of inspiration to fill in gaps in my story.
6) RELAX. Muses don't like to work under pressure. They like to be given candy and the freedom to run around. They do our best work for us when we're not wound up and barking orders at them, and the best way for us to do that is to make sure that we're in a good state of mind to be writing.
In conclusion, just remember...a happy Muse is a productive Muse. ^_^ The key is learning how to care and feed for yours so that it stays happy.