'No bootable devices found' error on my Dell Latitude 3480 Ubuntu 16.04 system.

I have a few months old Dell Latitute 3480 notebook with Ubuntu 16.04 (only, no windows) installed.

Today I was trying to boot the pc while having inserted a new usb mouse, during which the system gave an error of 'No bootable devices found'.

I tried to reboot several times, but with no success and I am stuck with my machine not starting. I would not like to format given all my data and software on the machine.