This morning I click on my thunderbird but got a small box with
message, part of it saying:'default' . I think I click on the word
under default.

Thunderbird opened but I had to reset my username, incoming server
etc. (I use AAPT)
I thought I could retrieve all my emails, folders and address book but
no it is now just a brand new setting with nothing.

I know that my setting and past emails are somewhere because when I
use Google Desktop I can click and read (in another format) my
'hidden' emails

Step by Step can you please tell me what I have to do (I am not
computer well educated)