Question Attempted installing video card, computer won't turn on anymore

Dec 12, 2018
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Recently I bought an Nvidia GeForce GT 710, and tried installing it into one of my computers, a Dell Optiplex 755.

Once slotted, POST failed with 6 long beeps, with a short pause between the 4th and 5th (video card failure?) but diagnostic lights 3 4 (Memory subsystem configuration failure?). Later I found out my PSU is capable of delivering only 18 A on 12V, and my card requires 20.

So I removed the card from the slot, and the same indications continued, even with the PCI-E slot now vacant, and the VGA cable back in the mobo's port.

Then I removed all the RAM modules and started re-installing them one by one. The audible beeps completely stopped, and the diagnostic lights changed to 2 4 (Video subsystem configuration or failure?)

I once again tried installing the card into the PCI-E slot and plugging the VGA cable into it, the diagnostic lights then changed to 1 2 3 (POST complete - On-screen error message?) but only a blinking pointer was displayed on-screen.

Removing the card and plugging the cable back into the mobo VGA port once again results in no beeps, and 2 4 on the dashboard.

At this point, I cannot get my computer past POST either way, with or without the card, and I have no idea what might have happened or what am I missing.

Thanks for any help in advance.