Thank you for your answer and support.
By playing with the buttons on the top of the chuwi, I finally found a way to reboot the system.
Now, the touch screen is working well again without any particular interventions.
Nevertheless, I have just a question regarding your solution:
As I did not buy the Chuwi keyboard, how can I change or update files on the tablet if the system is not working again in the future ?
Do you know if there is a solution (for example, "connection to a PC via USB" or "usage of a Bluetooth keyboard" or etc ?)
Thank you in advance