[server] Ubuntu Server 14.04.4 Server 64-bit - VirtualBox, Windows 10 host

Hi,
I have run into trouble after installing an Ubuntu guest on my Windows 10 host.

My laptop: Dell Inspiron 15, 5000 Series
Host: Windows 10 64-bit
Guest: Ubuntu 14.04
VirtualBox: 5.1.2

The installation of Ubuntu went fine, and I was able to play around a bit, but now I can't boot into Windows or Ubuntu. When I power on my machine, I see the Dell logo, and then a black screen with this message:

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

I ran the onboard diagnostics, and it says it passed all the tests.

Any help would be appreciated!
Thanks!