When you need to come up with an idea and you're drawing a blank, write something down. Even "the bad idea that I can't even come up with" is something you can change.
More often, you're coming up with lots of things - and they're all bad.
Pick the one you like the most (or - more likely - hate the least) and write that one down. Now you've got something, even if it's something bad.
Crap beats zip.