Upgraded to windows 10 and all my folders and events are gone and it's critical I have them.
My sub folders & events are all gone and I have 200 unopened emails that had previously been deleted over the past few months that are all unopened.
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I found the subscribe box, but it is greyed out.
Solução escolhida
You can check for a folder named C:\Windows.old\Users\your-username\AppData\Roaming\Thunderbird
If you do not immediately see AppData, make sure Windows shows hidden files and folders. To change that setting, see this article (hopefully it's similar in Windows 10): http://windows.microsoft.com/en-us/windows/show-hidden-files
Does such a folder exist?
If so, look for your old Thunderbird profile. For the default Thunderbird profile location see http://kb.mozillazine.org/Profile_folder_-_Thunderbird
If none of the above works the only way would be to restore a backup of your Thunderbird profile you created prior to the incident.