MathCad Re-install

You should be aware of: https://community.ptc.com/t5/Mathcad-Tips/Official-PTC-Mathcad-15-and-Prime-1-6-End-of-Sale-Announcement/ta-p/818870

If your computer didn't change (more specifically, the MAC address of your network interface is still the same ) your old license file, for which you certainly (should) have a backup might still be valid.

If not, your only chance is to contact PTC support for help. (This forum is NOT the PTC support portal/site or whatever.)

Success!
Luc