Check whether you can reinvigorate your passion. What made you stop loving it? Did you get distracted from your original goal? Did you over-complicate it? Did someone else solve it better? Be brutally honest with yourself about what's preventing you from getting excited and fix that.

If you find that you just fundamentally aren't excited about the problem them you owe it to yourself to move on as soon as is practical. Chalk it up as a life lesson and be more careful next time.

I've been there once and I think the matter is very delicate. Sometimes, you have to push through a terrible lack of excitement. Knowing if it's one of those times is hard.

That's true. There's a big difference between an emotional slump that lasts a short time and a big drop in passion that seems permanent.

I'll be honest, this is probably the primary fear I have that's kept me from even attempting something in startup-land. Even if I have an idea (EDIT: or discover an existing startup) that I feel passionate about now, I have no idea how I'm going to feel in X months from now. My passions are fickle and manifold.