I would thank you for your answer but I'm very surprised.
When today I've start up again the server it boots without aby problem and ANY
change has been done since last failed boot.
I've take a look to grub.conf and it seems that all is fine: boot disk, root location, boot image, ...
Really surprised but I can assure all people that grub>
appears on last day's boot!
Well, I supose that in all HW and SW there are goblins.
If anyone have experienced this issue before I'll be grateful if a new post is created on this thread. Probably with another experiences this strange behavior could be explained.