After upgrading my ubuntu server 15.04 to 15.10, it won't boot anymore. After starting the machine, it enters the BIOS. Selecting the linux drive to boot in UEFI mode just returns me back to the BIOS main menu instantly. Selecting it to boot in non-UEFI mode nets me a black screen with blinking curser.
Boot Repair tells me that grub (and everything else it might be checking) is in order.
The Ubuntu system is installed as UEFI and is the only OS present on this machine. The exact same thing happened on the previous release upgrade, at which point I just reinstalled Ubuntu get get stuff going again. Seeing as this might be a recurrent problem on my machine, I would be happy about any pointers as to where I could look for the source of the problem.
I am running this on an ASUS Motherboard, AMD CPU, Ubuntu Partition on a USB3 external HDD.
Big thanks in advance to any and all that might be able to shed some light on this.