When you install 8 after 7, is normally uses the Windows 8 boot menu, which is different from Windows 7. Do you get the page with the different options, or does it just go directly to Windows 8? If I remember correctly, you may have to select another OS from the menu, if you get one, and it will reboot the system into Windows 7.
But Pat's suggestion might get the system back so it will use the Windows 7 boot menu.
If you get a chance, could you open and Administrative command prompt and type bcdedit. Then take a snipping tool picture so we can see what entries are shown. Attach using the paperclip on the advanced replies.