I assume you've had jobs since then. What did you learn about the industry you're in? Where do you see problems that could be solved with software? And did you meet anyone on the business side of that industry that you think might want to be in a startup instead?
I didn't start from "I want to have a startup" and go looking for an idea. I started with an idea based on industry experience, and realized a startup is the way to get it done.