Problem is over.
I put the player aside and the next morning I was relieved to see that rockbox was in play-mode again !?
Even the time-settings were up-to-date .
The problem occurred when I connected the player with the PC while I was playing the music.
PC recognizes the player now so I'm out of trouble.
Thanks for your reply.
About the installation: I read the manual and then I followed a you-tube instruction. I didn't realize that the option "bootloader" in Rockbox Utility, when downloading the firmware, changed to "not chosen"; that's why I think that it isn't installed. Can this still be done ?