Sims Medieval refuses to open

Apparently, this is a very well-known problem happening on Windows 10 PC, running the disk version of the game. As stated on this thread on The Sims forums, you should try to convert your game key to a digital version from Origin:

I was having trouble running it on Win 10, too. I went into Origin and put in my product code and was able to convert them to a digital download. They run perfectly.

How to do this:

Go into Origin and under the Origin menu at the top, click on "Redeem Product Code". You enter the code from your physical copy there and it will turn it into a digital copy.

EA Community Manager advises to uninstall any version of the game before downloading and installing the digital version from Origin:

Please, try this:

• Uninstall both the disc and downloaded versions of the game.
• Use Ccleaner to clean your registry.
• Restart your computer
• Install the game again through Origin.