Saved in Keynote, now it won't open

[stupidmachines]

I'm making a presentation in Keynote; I saved it once (the way I normally saved it, I thought) and now it ends in .key and won't open.

Is there a way I can open this and change the format? Or change the format without opening it? It's for school and I've worked pretty hard on it and I don't want to have to redo it.

Thanks.