This morning, when I opened Thunderbird portable, I got a pop-up window asking if I would like a new e-mail address. The pop-up had a pre-selected (checked box) for something called gandi.net. I selected the button labeled "Skip this and use my existing email". I then got an e-mail account set-up pop-up, on which I clicked the "Cancel" button. I was then in Thunderbird with none of my previous e-mails present, and no account information. Yesterday, Thunderbird was just fine. How do I get my e-mails back? I have hundreds of them.
This can often occur if your prefs.js file in Thunderbird is corrupted. It can happen to the local or portable version but is more common on files run from removable drives. The emails stored locally are likely unaffected.
You can check out the files by exploring to the ThunderbirdPortable\Data\Profile folder. There should be a Mail folder in there with any local folders and POP mail as well as an ImapMail folder with any IMAP messages.
As for your account configuration, your best bet is to just grab a prefs.js from a recent backup. Even if the backup is a few weeks old, we'll just be getting the config file (prefs.js) not the mail files, so it won't affect any saved mail.
If you don't have a recent backup and there are no backed up prefs.js files in your profile folder, you'll need to manually recreate your accounts.
Additionally, be sure to scan your drive for errors if you didn't recently unsafe eject or similar.
Sometimes, the impossible can become possible, if you're awesome!