Its an OEM computer. The manufacturer might have set it up to not allow installation of aftermarket devices like a new graphics card.
Remove the 750Ti, put back the old card (if it had one) and try to boot again. If it works you might want to first uninstall the old graphics drivers, before trying to switch the card again.
And in case you didn't know, have only one graphics card installed. Having multiple cards can cause problems.
1. i can run my friends 680 perfectly and 2 i have uninstalled the drivers