If Ubuntu crashed and Thunderbird lost all accounts on your system, you can try the following steps to restore your Thunderbird accounts:
Open a terminal: Press Ctrl+Alt+T to open a terminal window. If you can't access the terminal directly, you can switch to a virtual console by pressing Ctrl+Alt+F2 (or any F-key from F1 to F6).
Stop Thunderbird: Run the following command to stop Thunderbird processes: