Once Onboard Graphics Are Disabled, What If New Video Card is DOA?

I decided to upgrade to a separate video card in my HP p6710f PC with ATI Radeon HD 4200 onboard graphics. The card I purchased is an MSI Radeon R7 240. I installed the card and although the fan ran there was no video output. I don't know if the card is DOA or another problem. If I disable the onboard graphics in device manager or in the bios before installing the new card and the new card is in fact DOA, how can I ever regain output from the onboard graphics again for output to the monitor? It seems to me I would be dead in the water with a black screen and no way to do anything. If anyone can help and knows the answers to this problem it would be greatly appreciated!

You need to disable onboard graphics after installing a video card and starting the computer for the first time.
Also make sure the video cable is properly connected to the new card.

After installing the new video card and I had no video output so I couldn't disable the onboard graphics. If I moved the video cable back to the onboard graphics then I would have a display back. Suppose the new video card is actually DOA. I leave the onboard graphics connected, then boot up, disable them and shut down. I then move the cable over to the new video card, try to boot up, but there's no display because the card is DOA, won't moving the cable back over to the onboard be in vain because I've already disabled it?

I think the onboard card would work if it's disabled and no other cards are installed. I'd have to look at your motherboard manual.
Is the MSI Radeon card new or used?

The Radeon card is new. I figured it was DOA so I sent it back to be replaced with the same card. I've installed other cards in PCs before and never had this problem. I want to install the new card as soon as it is delivered, but now I'm concerned because I don't want to uninstall the onboard graphics first, only to find the new card doesn't work and I'm left with no display after disabling the onboard graphics.

addRam: If I could get things back to default should I run into a problem by just pulling the motherboard battery for 10 seconds it would save the day. In that case is it alright to disable the onboard before installing the new card?

I'm going to try it by disabling the onboard first then. Yeah, Jersey. That's my home state! Thanks to everyone trying to help!!!