Type or copy & paste this command %appdata% into the Search Box and press Enter:
It will open your Roaming folder where you can find the "Thunderbird" folder. It is the location of a Thunderbird profile and since the current profile doesn't work, you can either delete "Thunderbird" folder or rename it to something else, for example "Thunderbird-old"
Start Thunderbird again and it will recreate a new profile. Then, re-add your e-mail account and all should be now OK
Samsung Galaxy Note 7 review: An almost flawless smartphone, almost