Writer's block happens to everyone. Even the most prolific writers stare at blank pages sometimes. The difference is they know how to push through.
When you can't think of what to write, just start. Write anything. "I don't know what to write" is a perfectly valid first sentence. Often, the act of writing itself breaks the block.
Change Your Environment
Sometimes a change of scenery helps. Go to a coffee shop. Sit in a park. Different environments stimulate different thoughts.
Or change your medium. If you're stuck typing, try handwriting. If you're always writing at a desk, try lying on the couch. Small changes can jumpstart creativity.