If you have accidentally deleted one of your mail accounts by clicking on the "Remove Account" button in Thunderbird, this does not mean that your mail has actually been deleted from your computer. That mail should still exist in a child folder in the Mail folder in your profile folder. (This assumes that the account is not using the Global Inbox, in which case its mail would be stored in Local Folders.)

Once you've gone to your profile folder and verified that the mail for the deleted account still exists, do the following to get it back into Thunderbird. You may wish to make a temporary backup of your profile folder before proceeding.