Thanks for your help.

I made a complete reset (purge) of  xserver-xorg, nvidia
and by nvidia-uninstall.
I choosed the nvidia driver free.
At reboot, victory ! It works.

Many configurations were lost by buster,
long time to return them.

I think (maybe) that the non-free driver of nvidia,
can not work with buster or the buster kernel 4.19.

Regards,

Andr�