I’m using Cubase 12 in Windows 10. I didn’t have this problem when I first started using it. The program freezes up when start it. The program loads but I can’t do anything past that: load an existing project, start new projects, or even close the program down. I have to use Task Manager to shut it down.
Best place to start is with Safe Mode.
This will start Cubase with all preferences at default and plugins disabled. That will narrow down the troubleshooting.
If that works, think about any new plugins added, or anything else new to the system - they may be playing up. Disable them and try a normal startup.
I also had that problem with Cubase 12 Pro and Win10. Tried about everything but it kept on freezing most of the time during startup.
What finally solved the problem for me, was checking my Windows Logs for errors and warnings. I had the 10016 error, which prevented Windows from accessing a certain file. And while I was trying to fix that problem, my Cubase began starting up normally.
There are several logs in Windows Logs. Especially check under Windows > System log for errors.