Apple Mail - doesn't see accounts, all gone

Suddenly, opening mail is as if never used before, won't let me past a dialogue box to add a mail account so I can import existing ones... at a loss as to what to do.

Not enough to go on. Please read the post in my sig and post again. Don't forget to add to that post any recent changes to your system, even if you are 100% confident it had nothing to do with mail.