Looks like your file association somehow has gone wrong.
As that's something which usually is set correctly by Primes installation procedure, you may try to uninstall Prime completely and then reinstall it again.
You may also try to set the desired file association manually yourself. You may find instructions how to do it by searching the net - don't forget to also add the Windows version you are using - e.g. setting file associations in windows 11 - Google
One easy way to do it is to right click a Prime mcdx file in Explorer and chose "open with". If Prime 10 is not offered in the list scroll down and click "use different app" until you see something saying "look for app on PC". Click and maneuver to the exe file of the current Prime version, usually located at "c:\Program Files\PTC\Mathcad Prime 10.0.1.0\MathcadPrime.exe". Choose this file and don't forget to check the box saying something like "Always open mcdx files with that app" .
So after doing so you should again be able to open mcdx files by double clicking them.
Letting Prime the installation program do the job might be the cleaner solution - it also will add the file association for the template files.
I am not sure if Prime is able to do a repair installation = running the installer without uninstalling the program first and it will just repair things like broken file associations etc.
