IBM Server/EFI Boot: What could have happened?

[Jorge Andres Brugger]

I have an IBM System x server with Ubuntu Server 14.04, standard 
settings (UEFI BIOS defaults, LSI RAID controller, two mirrored disks).
LSI config said RAID is "optimal".
Yesterday I rebooted it and refuses to boot because "missing operating 
system". After fighting for a while with the problem I managed to 
explicitly configure the boot using /ubuntu/grubx64.efi and the server 
is alive again.
This was not configured this way before. In the past I´ve rebooted the 
server many times without any issue.
What could have happened? How I can do to return to the previous 
setting? (i.e. "standard" and non forced EFI boot config)
Thank you very much!
-- 
Jorge Andrés Brugger
Departamento de Informática
DASU - Obra Social del Personal de la Universidad Nacional de la Patagonia
Comodoro Rivadavia, Chubut, Argentina
Teléfono (0297) 446-4444 int. 103
Correo electrónico: jbrugger at dasu.com.ar
Website: www.dasu.com.ar

"Quien hace, puede equivocarse. Quien nada hace, ya está equivocado." 
(Daniel Kon)