Take A Break
Sometimes you have to take a break and relax and give yourself time to cool down. Take a minute to de-stress yourself.
Read a book. Watch a movie. Listen to Music. Anything that might kick start your imagination again. It's okay to get inspired by other people- just don't steal their ideas completely.
Write and write and write and don't stop for five or ten minutes straight. Doesn't matter what you write, just keep writing. Your brain might jump-start from the exercise and motivate you.
Writer's Block can be caused by a lot of things, but if you become frustrated at yourself, it will just make it worse. Don't be afraid your book is bad- remember it's a first draft.