You must be able to think of something. Just ask yourself "What are the major pains in the ass I deal with on a regular basis, and can any of them be solved with technology?" It's probably better to go in with a high-concept, far-fetched idea than no idea at all. Just see what you can come up with.
I have this great idea for a dating site ...
Funny you should mention that, I'm building this semantic search engine...