Hello

It happens occasionally in a sleep mode. PC wake spontaneously with such message:

No bootable devices found.
Press F1 key to retry boot.
Press F2 key for setup utility.
Press F5 key to run onboard diagnostics.

Pressing F1 doesn't help. F5 diagnostics shows no SSD found. F2 never tried 

Long press power button to reboot turns it back to normal condition.

What could it be? Are any suggestions?

Device: Dell Latitude 11 5175, 8gb RAM, 256gb SSD, Dell Latitude Keyboard