Thunderbird has a nasty habit of once in a great while forgetting about the existence of a profile if it uses the default name. When this occurs, you suddenly start up in the new account wizard and it looks like you lost everything. You haven't. It's just lost track of your profile, which is probably intact.

Another possibility is that your prefs.js file is empty or corrupt due to either Windows or Thunderbird crashing. Your prefs.js file has all of your account information and settings. In this case you haven't lost any of your mail or address books, but Thunderbird no longer knows how to fetch new mail or send messages, or display the folders in the folder pane. You'll need to recreate the accounts in Thunderbird unless you have a backup or unless it created a copy with a different name (for example, prefs-1.js).