NUC5i3RYK doesn't want to boot on my M2 SSD drive

Hi,

I've got a NUC5i3RYK unit with 2x4GB DDR3L-1600 RAM and a M2 Transcend TS64GMTS800 SSD drive.

I've installed on the SSD Drive a Windows 7 Pro without any problem. I've let the NUC on a table for a moment (1 month) and when I restart it today, it doesn't want to boot on the SSD Drive, arguing that no bootable media was found.

I've taken another NUC5i3RYK unit and placed on it the SSD drive and the RAM drive and then power it up, and it boot on Windows 7!

On the faulty NUC, I've upgraded the BIOS, but it doesn't change anything. I've compared the BIOS settings on both units, and they are the same.

I'm really lost. Was someone placed in the same situation?

Many thanks in advance,