Thank you for the alternative and I may use it to further customize my screen display but I’ve just decided to reinstall Zorin completely.
First, I installed it without the graphic drivers for nvidia. So, I installed them afterwards. Since there where many options under : Software Updater | Settings | Additional Drivers, I didn’t know which one to choose and therefore I certainly have installed not the best one which as a result might have been the cause of the problem.
Then, I reinstalled Zorin with the graphic drivers for nvidia and it’s working perfectly.