I'm very sorry to hear this. I hope things work out for you. My suggestion would be to just say that you went to the startup as it sounded interesting but with

I'm very sorry to hear this. I hope things work out for you. My suggestion would be to just say that you went to the startup as it sounded interesting but with no real expectations. You tried it for a year but it wasn't really for you. If you can then say something about the new company that attracts you to it, implying a contrast, so much the better. As others have said, the key thing is to be general and move onto something you want to talk about as soon as you can.