This is an issue that I believe to have solved- it is like this problem here -
Dual Boot Windows 8 with Ubuntu 12.10
My suggestion (and it's only a suggestion) is to tpe "msconfig" (without the quotes) on the Windows 8 start screen. In the boot tab, see if Ubuntu is listed. If it is, select the entry by highlighting it and click on "Set as default". This should hopefully load the Ubuntu bootloader instead of Windows' boot menu allowing you you boot into Ubuntu.
Also, from the start screen type in "uefi" and in the settings option on the seach charm, click on "Advanced start up options", click on it. Scroll down to the bottom of the entry and under "Advanced start up", click Restart now. You should now have an option to change the boot order.
I know it is the same solution to another one but to me the core problem is the same- Windows 8 bootloader
Hope this works!