In most cases, unless you changed it yourself, the mendix models can be found in C:\Users\YourAccount\Documents\Mendix
If that’s not the case, then
Still not found. My guestimation is that you don't have it available anymore.
My last check would be the recycle bin.
It is unclear to me if you still have a local copy of your project. How then did you upgrade? You can always download a fresh copy from the teamserver. That will be the latest version committed to the team server. Now if a collegue did an upgrade of the Mendix version you need to download that version also and just open your local copy with that version. It will then upgrade your local copy also and you could then commit your local changes. If you no longer have a local copy then you are in bad luck because changes will only be saved if you commit your changes to the teamserver.
So could you elaborate on what you have done in the upgrade proces that made your local files dissappear?