There are times when something is really wrong, and you just absolutely cannot write for weeks or months at a time. I'm not talking about those moments today. I think something more common is moments of mini writers block, and those happen several times a week, at least for me. These are more just bumps in the road that you are constantly having to drive over, and here are some things that may help when you hit one of them.
Revel: After I've watched an incredible movie or show, or something exciting is happening, I usually can't make myself focus enough to write. If this happens to you too, just let it happen! Get your writing done before exciting stuff happens, and then just enjoy life as it comes. Its ok if you can't write for a few hours, or until you've slept it off, because it will wear off and then you can get back to serious writing. But in the meantime, revel in the excitement. Learn from it. Your writing will benefit from it.
Think Positive: Often it can go the opposite way, and something so frustrating and negative can happen that you just are too upset to write. I'm not talking huge, life-changing crisis, I'm thinking about all the little annoying, depressing things that happen all the time. When this happens, give yourself a bit to cool down. Talk to someone who always makes you feel better. Listen to some great music. Read, and let yourself forget about it for a while. Eat some chocolate. Then go back and see if you're ready to write.
Revise: Sometimes a project just isn't going the way you want it to go. You just get stuck. Try going back and pin-pointing the spot where you got off track. Go from there, and see if things flow more easily.
Relax: Yes, there are genius writers out there. No, you may not feel like one of them. But so what? Everything can't be written by Shakespeare. There are stories only you can tell. Don't let yourself be overly intimidated by genius writers, because they will always be there, and you will probably always feel intimidated by them. Don't let it keep you from writing. Learn from them, enjoy them, envy them, but write!
Write: Sometimes it just takes sitting your butt down in the chair and writing the next sentence. It may not be great, but thats what revision is for. Just do it, just get the words out there. You may only need one sentence to get you back to your old writing self. So just write!