You're saying that you can't even find the thunderbird client? I will assume you use Windows and my suggestions follow that assumption.

1. enter %appdata% in search box in task bar and press enter
2. that should open file explorer at appdata\roaming
3. locate the thunderbird folder and COPY it to somewhere else (this shouldn't be needed, but just being safe)
4. download thunderbird from thunderbird.net, preferably the same release you were using (either the ESR or Monthly release) and install
5. start thunderbird and it should open to the proper profile

This assumes that the profile itself was not corrupted. If the above doesn't work, please provide some specifics. thank you.

Hello,

  Thank you for your reply. Using your suggestion of %appdata% I found the Thunderbird file and copied it. It had 3 files in it and 2 other things. The files were Crash Reports 624kB, Pending Pings (empty) and Profile which was 1.60GB.
   When I searched 'C' drive I could not find these. Only some install exe and files from 2023,2020.

  Did what you suggested but it did the same as when I went to install Thunderbird again and wanted my e-mail and password, thought I better not incase it wiped out my previous set up.
   Went through with it this time, downloaded all new e-mails but I do not have any old e-mails or sent e-mails or contacts or any of the set up I did have.

   What happened was I went to upgrade to 150??? or something and it said to restart, so I closed Thunderbird and then started it up again but the short cut on the task bar when I clicked it said along the lines of this short cut no longer works.
    So I thought maybe I should have restarted the computer, I did that and the same thing happened, short cut does not work or it is in a different place, something like that.
    So that is when I tried to install Thunderbird 78.3.1 exe again. The original I got and it did the same thing I described above, wanted new set up (e-mail,password) done.

   So anyway do not not know if any of that was relevant but any suggestions how to retrieve my old e-mails and contacts etc?
  Any suggestions much appreciated.

    Terry

Being prompted to enter email address again indicates thunderbird had problems with the old profile. To clear and restart this process, I suggest this:

1. as before, locate and copy the appdata\thunderbird folder to a safe place. do this with thunderbird not running
2. uninstall thunderbird
3. download and install the ESR version from https://www.thunderbird.net/en-US/thunderbird/all/ that is version 140.
4. start thunderbird
5. if prompted for an email address, skip it, and click help>troubleshootinginformation
6. then, scroll to 'profiles' and and click 'about:profiles'
7. the next screen will list all profiles. Your old one should be there. see if you can launch it and let me know status.