I think this has only a 50 50 shot at working but you can try doing a system restore to before whatever happened to screw things up. Its easy to do though instructions below
otherwise copy the mbx file that you want to safe folder
then re install the eudora and then copy the mail box over the new mail box that it creates on install.
System restore instructions - windows 7 or 8 8.1
hold the windows logo key down and hit R
type rstrui.exe in the run box that comes up and hit enter
select restore to an earlier date.
Under the "suggested restore" which is usually not old enough to help is a link for "OTHER RESTORE POINTS"
click that if the suggested ones are not BEFORE THE PROBLEM STARTED
choose a date that is BEFORE you experienced the problem
hit ok / restore
the computer will start to shut down
a restoring settngs box will appear with a status bar
when it finishes your computer will restart with the same settings it had on the date you restored to. your data will not be affected.
It is not perfect. But it fixes a lot of things.