Writing, drawing, worldbuilding....anything creative has its ups and downs. The good times are when you have so many ideas you feel like you're rushing to get them all down. Furiously scribbling while the muses smile on you. There's nothing like riding that wave of inspiration! Be sure to say "thank you" when fate does bless you with those moments because sometimes they feel few and far between.
That being said, let me move on to the real point of this post: what to do to pull yourself out of a rut.
First of all, be mindful of your feelings, your thoughts betray you. Doing something creative is a great way to escape into something fun. But trying to force it is about the worst thing you can do. For a time-killer, poops-and-giggles guy like me that's not as much of an issue as for someone who does this for a living. I can't think of many jobs cooler than getting paid to do this...but what happens when the laughter stops and it becomes the daily grind of work? Again, I'm rambling. The take away is this, don't count on just being able to plop down and churn out something. It doesn't work like that. It really really doesn't work like that if you're in a bad mood, tired, frustrated, etc. You can improve your mood with this stuff, but don't think it's a cure all.
Second, find what inspires you. If you can't think of something, go find something. I like landscapes (maps especially if you haven't figured that out yet) and concept art. It's pretty easy for me to look at an image and "fill in the gaps". What is this place? Where? What's around the next bend in the road? What lies at the end of the journey? What started it? What are the people, creatures, modes of transportation like? In a specifically fantasy sense, what are the dangers of the world? What is the political system/religion? Are those two separate? Is there magic, what are its rules and limitations? Armor and weapons fascinate me. What does each piece mean/do? Why did the artist/character choose what they did?
Another suggestion...READ! Find a magazine, book, blog, website, forum, guild, whatever. I like to take an empty backpack to the library and just wander. If you're a worldbuilder it can be overwhelming how much you have to consider to flesh out all of the details. Don't try to see it all at once. Focus on bits. Mythos, philosophy, language, commerce, weather, terrain, etc. I say that like it's such an easy thing - believe me, it's not. I get stuck in a funk at least once a month. It happens. Shake it off and move on. We can't be demanding of our imagination, it often does whatever it feels like.
One last bit of advice. Don't erase, apologize, or beat yourself up about it. I used to try to make everything fit into one world. It didn't work. I have pages and pages of notes that I don't know what (if anything) I'll ever do with them. But it was fun to write it at the time. And it's fun to go over it again later. Sometimes you forget something really great hidden within an old notebook and it's really cool to rediscover it - even moreso when you can remind yourself "I did that. It's bloody brilliant!"
**Thanks to Nathan (aka Vorropohaiah) for mentioning documentaries. I knew I forgot to include something! I watch a lot of documentaries. Science, history, cultural, space, etc. They are almost guaranteed to stimulate the imagination. And even if you don't get a lightning strike of inspiration, at least you're learning something that you can draw upon later.