What do you do when you lose interest in writing?

I would explore why you've lost interest. It might be that you're just going through a tough time or something is happening in your life that you need to focus on right now and your creative energies are going toward that (this could be totally unconscious). So you're not actually disinterested in writing completely but just focusing on other things at the moment. It might also help to think small - that is, do a small writing exercise every day or every few days or journal or write a poem (if you're into poetry) without any thoughts about what might happen in the future but just as a way to get your own feelings out and keep your creativity going. No commitments, no pressure, grammar and spelling can be crappy, no one's going to read it. It's just for you :-).
.
I agree with the thoughts of Monica and Tam. I understand it is not easy to get back on track once lost interest but if it is in your mind and like a goal, you need to do one thing-- Start now. Start writing anything you like or want. And every day write something. Things will get better. We all are with you.
.
I took a break from writing and focused on other things instead, like reading and other things I enjoy. It helped me.
.
I have the exact same issue. I know what I want to do with my stories but I think I'm just not in the right emotional state. Can't much deal with it- real life can be such a bugger sometimes hey.

But keep those ideas in your head for the day you really want ti start writing again
.
I didn't write a single thing even though I had ideas, nor did I read one book for a whole year. My brain went on strike. Everything I tried to read I put down, I just didn't want to do it. And honestly I feel a lot better. I lost joy in writing and taking that hiatus that my subconscious knew I needed did the trick. I just started writing again a few weeks ago.