Windows

You don't have the cable plugged in correctly, or the graphics chip has taken this moment to die entirely.

"I enabled my on-board graphics before restarting"

So there is onboard <i>and</i> a separate graphics card? Otherwise, why would you need to enable onboard graphics, right? If so, make sure the monitor is plugged into the right graphics port.

If you set a fatal setting in BIOS, you typically can clear that with a jumper on the MOBO or pull the CMOS battery for a little while.

It's also common that the MOBO video is analog and the video card is digital. Two different connectors, obviously, but some monitors have to be manually switched between analog and digital inputs, or sometimes just power-cycled to do that.

Removing the Plug In Video Card. Typically if these are not correctly inserted in their socket or in the case of AGP Cards the computer gets moved around the Video Card can work it's way out slightly and you get no Video Display.

So the answer here is to re-seat the Video Card or remove it to start sorting the problem.

I would also look inside the Video Plug for Bent Pins which can also happen occasionally.

But as said above if you have made a Fatal Error in BIOS all you need do is change the Jumper to Clear or remove the Battery. If you remove the Battery remember to first unplug the Power Lead and allow the LED's to go out so you don't do any damage if you drop the battery. Metal Cases Batteries cause Short Circuits and damage to M'Boards with Power.

To reset the BIOS all you need do is enter it set the current Date & Time and then Load Defaults. Save the changes as you exit the BIOS and you're off and good to go.

With most M'Boards when you plug in a Video Card the On Board Video gets turned off.

Col

had to ask...also check and see if the monitor is bad by swapping with a different one.
sometimes they do funny things. i even had a bad power supply cause a similar issue.

First I would check if the mobo has an onboard video card, then I would just simply remove the old video card and attach your video cable to the onboard. By default the onboard will work without any bios settings.