Czuch: Can you not remember the file name you gave it when you exported it (i am guessing you did Export it lol).

If not then, click Start -----> Find -----> all files and folders ----->(now in box that says 'All or part of filename' enter) .LDIF ------> click Search

Hope fully that will find it. It should be a .LDIF as that is the default Thunderbird saves it as, and should be the only one you have

Hopefully when you know where it is open Thunderbird-----> Address Book -----> Tools -----> Import (then navigate to where you hopefully found the file)

Lets hope that solves that little problem