Some people find that it helps them to have a writing prompt to start a flow of ideas, so I thought I'd share one that I've found useful:
Get a newspaper and leaf through it until you find a headline you find intriguing.
Do not read the story!
Instead, make you your own story: the who, what, when, why, where, and how that's not there in the headline. Obviously, the best headlines for this exercise are the ones that don't give too many details. And if you suddenly have a great idea that doesn't fit the headline, go with it, the whole point of this is to get your imagination fired up.