Yes, I know... It's hard. You're sitting before a blank sheet of paper -- or more probably a blank Word document. You just can't think of anything to write. It happens -- to all of us. In my case, there is nothing that makes me happier than just writing effortlessly once the creative juices get flowing. But getting those juices flowing, sometimes that's the difficult part. But never fear, I have some tips that might help you if you are struggling with writer's block.
First off -- and this sounds obvious -- but there are a lot of books on the market that have writing prompts that can help spark your inspiration. I never used them when I used to face a lot of writer's block, and it's a shame, because the resources are out there. You can search such books at your favorite bookstore.
Another idea is to attend a writer's group like the one I attend. There, participants are given writing prompts, and then we write as a group (on our own, of course). But the idea is that you are basically "forced" to write. Sure, you don't have to if you don't want to. But I will tell you that having the time pressure and being in a group atmosphere -- it has forced me to break through any thoughts of "but I don't know what to write," and just get the words down on the page. It doesn't have to be perfect. You can always go back and fix what you've written later. But the idea is that you learn that there is never an excuse for not having any ideas.
Perhaps the most valuable thing I've developed, though, to help with the writer's block is a firm understanding of story structure. A lot of times what used to happen to me before I understood story structure was I'd have a "feeling" about something I wanted to write. What I mean by that is that I wanted to write something sad about two characters. Or something comical. I'd have an idea of what characters I wanted in my story, but I didn't know what to do with them before I understood story structure. I might have an idea for a great character, but I didn't know where she should go. I would write twenty pages or so, but then get stuck. Sure, I introduced my great character, but I didn't know what to do with her after that.
Then I would have leave everything up to inspiration. Trust me, I still need to be inspired in order to write. I write best in the morning -- that is when I feel most inspired. But now that I have a deep understanding story structure, I have a framework (not a formula) in which to place my characters. I now have the tools necessary to outline my story on a conceptual level before I even start writing chapters and/or scenes.