Hi, I've got 2 Probooks which have this same problem. They suddenly reboot and say - no bootable disc - insert disc and press any key... They do this rarely, but it's kinda problem. Reset the BIOS to factory settings or try to update the BIOS? Thank you.
I've updated the BIOS to version from November 2016 and tested HDD with HDTune, which showed 100% health and 100% in surface test. Will see if it works.