Search your harddrive for the file mirc.ini
The program "Everything" can help. https://www.voidtools.com/downloads/
If you find two or more mirc.ini files, then good, that likely means the older file contains all of your settings.
Make backup copies, then try to transpose the good mirc.ini file on top of the brand new mirc.ini file.
Or, figure out why mIRC is not finding the correct mirc.ini at its correct location. I have a whole article written about this, here
mIRC does create backups every time you do a new install on top of an old install. The feature is turned on by default in the installer.
You might find a good recent copy of your mirc.ini (and other .ini files, ie, servers.ini) located there.
Make sure that mIRC is not running
when you move files around on your harddrive.