In my instruction I assume that the profile is located in the default location. if you changed this default to some other drive for space reasons for example, you will need to correct my instructions so your looking in the right place for your old profile.
First navigate to the Troubleshooting information on the help menu. Scroll to the bottom of the "Application Basics" information and select the last entry. About:profiles.
This heading is followed by all the registered profiles Thunderbird currently has registered.
If none of the profiles listed here are your missing one, you will need to move on to creating a new profile that is in the same location as the old profile. Use the "Create a new profile" button at the top of the about:profiles page to start Thunderbirds Create profile wizard.
Lots of information here, but just click Next.
Now click the choose folder button. This will open a standard open dialog in the default location of the profiles. What I see looks like this
You will be returned to the about profiles page and the list will be refreshed with your new profile name which will show as default. A restart of Thunderbird should see the correct profile launched.