Same exact thing happen to me as the original poster when I installed ubuntu. This helped me. Shut your computer down and boot to bios or uefi boot menu. You might see 4 options:
ubuntu
Ubuntu
EFI Network 0 for IPv4
EFI Network 0 for IPv6
Select the Ubuntu with the capital U, thats what it was for me. One ubuntu is the actual ubuntu and the other is windows 8.1. Hope that works.