I often find that if I've hit a "writer's block," it's either because (1) I'm being lazy and don't feel "inspired" to work on my writing, or (2) I really have lost the inspiration for writing my book.

To parry the first one, you can use a number of techniques such as:

(i) Talk about your story with others. This will force you to summarize it down to its core concepts, the ones that got you eager to write your story in the first place, and will help you regain that enthusiasm.
(ii) Force yourself to sit down and work. Though at first it may be quite painful to eke out a single word onto your blank screen, once you get going, the process gets progressively easier. There are a few tricks to help facilitate this event, such as starting on the next scene before you stop writing for the day so your mind will already have something to work with on the next day, or writing about other things before you get back to your story. Just make sure you stay away from distractions (the internet being one of them).
(iii) Read about topics that inspire you, particularly if they're related to your own story (articles, blog posts, books, and even books on writing). Just be careful not to let yourself use this as another excuse to completely procrastinate. Once you feel your creative juices flowing again, drop what you're reading to get back to what matters: your story.

To get around problem number (2), you may try the following:

(i) Plot out your story beforehand. I found out, when working on my first novel which I'd decided to write as a pantser (as opposed to a plotter), that I'd written myself into a corner. That is, I'd gone off so many tangents, I'd lost the main plotline and actually bored myself out of writing anything more. Many famous authors talk about having written the book that they've always wanted to read. Well you should trust your own instincts too! You've read plenty of books, you know (consciously or subconsciously) what works with you, what gets you excited. It's the same with your own story! Which brings me to the next point...
(ii) Go back to the last chapter/passage/paragraph/sentence that really got your bloody pumping, discard everything you've done after that (if you're squeamish about doing this, you can save the rest in another file), and start writing anew from there on (it's like your own save point in a video game, and the best part is, you've got as many lives as you want!).
(iii) Remember to have fun. Sometimes your seeming boredom is actually fear disguising itself as such to prevent you from ever finishing your work. And the best way to vanquish it, is to say F* it and have fun with it. Write whatever you want, what makes you happy, no strings attached. Because you know what? There are no strings attached! The great thing about the digital age is that there is no cost to writing and rewriting as many times as you wish (except for some electricity to keep your computer's battery running and your own time), and that's exactly what you'll get to do. After you're done with your first draft.

These are just a few ideas to play with to get your writing mojo back, but there are tons of other things you can do (as evidenced by all the other answers to this question). Just remember that your creative brain is like a muscle: the more you train it to work and come up with ideas, the more easily it's going to come up with them.

Now go forth and conquer!