First off - what method did you use to go about turning your graphics card off?
Second - A few issues could cause this but normally it has to do with either power or heat in one component or another. I'd remove all unnecessary components from the computer such as graphics card, all RAM modules except the one in DIMM0, any HDDs/SSDs (Just unplug these), and all additional fans except the one atop your processor. Then see if the computer will boot up and state it's missing your HDD/SSD OS drive.
If the machine boots at this point then add one component back into the system at a time until the error occurs again and that will help you to determine what the root cause component is.
If it still won't boot then you need to look at a few things:
1) make sure there isn't any excess thermal paste on your processor that has excreted from the sides or flowing down toward the ZIF socket (processor socket). If there is then remove the processor, clean it off with an ESD (Electrostatic Discharge) safe cloth then replace the thermal paste in moderation so it doesn't repeat the same issue and attempt to reboot.
2) Grab a multi-meter or a volt meter and measure the voltage on the power supply by using my power supply walk through as a guide. (http://www.tomshardware.com/faq/id-1936217/install-power-supply-including-basic-cable-management.html)
Start there and get back to me if you continue to have issues.