I started to have the same issue on a custom built pc for windows 7 ultimate. I installed Zorin 15. And After 15 min of inactivity the screen goes black and the sound works and the cursor moves.
However, ctrl+alt+f1 takes me to the terminal. Ctrl+alt f8 does not take me back to the desktop.
I've also made sure to disable screensaver since it never works. As well as power settings. All are set to do nothing.
Please advise. TIA!