I don't think there is such a word as 'unbreak' although I know it was used in a Toni Braxton song 'Unbreak my heart'. Perhaps it is modelled on the expression 'unchain my heart', a Joe Cocker songer. Don't break my heart - yeah, well how many songs do you know that use this line?

You can't "unbreak" something, so you would never say that word. You can only "fix" something that is broken. "Don't break" is what you would say BEFORE something gets broken. For example, if someone is carrying a fragile plate, you might say "Don't break the plate!" so that they are careful. But if they drop the plate and it breaks, you can't "unbreak" it. You would have to fix it by gluing it back together for example.