In thunderbird, the addressbook is separate from accounts. That is, deleting an account has no impact on the addressbook. You would lose the addressbook ONLY if the profile was also deleted, which is never recommended. A normal process would be to add the new account, then remove the old account. if the person helping you deleted everything prior to setting up the new account, then it's gone, but if the person decided to create a new profile, although not needed, the old profile may still be there. You can check this yourself:

1. click help>troubleshootinginformation
2. scroll down to 'profile folder' in left column and click 'open folder'
3. you will now be in Windows File Explorer, click the back arrow once to position you at the 'Profiles' folder
4. if you see more than one profile listed, click to review each of them. The addressbook's name is abook.sqlite. If the file in one profile is larger than in another profile, that is probably your old addressbook.
5. you can retrieve it by clicking tools>import>import from a file. then click for addressbook and then click for sqlite file and then locate and se;ect the abook.sqlite file that you located.

"If you do a search in the c:\users\<yourid>\appdata\roaming\thunderbird\profiles folder, you may find abook.sqlite and history.sqlite. These are the technical filenames of the addresssbook and the collected addressbook. You can import them with tools>import>importfromafile. This sometimes happens, that thunderbird creates a new profile and some content is left behind. No, this does not happen to everyone and is likely a glitch wherein an update was interrupted by another program, such as an antivirus product."

This might help. search for "abook.sqlite". Found it in a file, C:/windows.old. look for last oldest.

Oh, forgot to add. then import to address book.