Hi,
here’s the Cubase Artist manual on freezing/unfreezing tracks.
Don’t worry, freezing a track is not permanent or destructive, you can definetly unfreeze your tracks again in Cubase Artist.
If you encounter any problems please share at which step the unfreeze process didn’t work - I am sure that there’s a solution to your problem 
Here’s another manual for Cubase Artist 11 - it shouldn’t be too different from the previous one.