A trick I always use is to take a really big subject and write about it with an impossibly strict word count. A couple of examples might be:
- What is the impact of deforestation on the climate in 50 words
- Explain how the combustion engine works in 25 words.
That sort of thing. This exercise does two important things. It forces you to write your way through the block and by being so restrictive, the relief of writing without such strict constraints once you're done will make your normal work seem so pleasurable, you'll forget about the block altogether.
It works for me anyway. I hope it works for you.