Computer won't display anything

I know this will sound really obvious, but I mean it can't hurt just to throw some ideas out there:
Check to make sure the monitor's power cable is secure on both ends
Make sure the display cable between the PC and the monitor is secure on both ends
Make sure your GPU (if you have one) is secure in the PCIE slot and any power cables are plugged in
Make sure the monitor is on the right input
Make sure the GPU power cables, if there are any, are plugged into the PSU securely

That's all I can think of haha. I'm not trying to demean you with some of those obvious answers, but hey, we all make careless mistakes

Again, just throwing out ideas here since they can't hurt.
Check the graphics card's HDMI port for dust/debris? Blow into it, carefully scrape in it with an extra tooth brush, etc. But I guess that wouldn't be likely since you tried the onboard HDMI as well and that didn't work... Can't hurt to try, though
Did you try the stuff I listed before?

In some cases the back plate or support plate, can have, depending on what its made of.
a Plastic clear cover or color one.
You must check if this is the case if it was made of metal ect.
With some you get the sticky plastic strip to cover the back support plate, for the back side of the board. already fitted or you have to apply it before fitting the back support plate.
If you fail to check this, when a metal type plate then you will cause a short.
And damage the board. from the back side.

If you have a good quality Psu it will detect the short, and prevent the Psu from completely powering up due to the short being detected by the psu, and its protection circuits built in.
And that is why you get no power at all from the Psu, short circuit protection.
Its saving your bacon, and board due to the short, possibly created by adding the plate witch is the last thing you did, after your system was working fine simple logic.

So basically it tells you what you need to look at.

So out with the board and if the back support plate was made of metal by chance not plastic. check its insulated on the side that touched the board.
If you have a cheap Psu then there is a chance you may of fried your mobo, i`m sorry to say.
But as a rule of golden thumb, what was the last thing I did before it stopped working. is a sure fire way to sort the problem. If you have a quality Psu, and find the back plate is metal touching the back side of the board but apply the insulating stick plastic protector. the board should work fine once done.

To reset the power protection circuits of the Psu.
First if the psu has a rocker switch on the back side of the psu set it to off.
Unplug the power cord from the wall, count ten seconds then plug it in switching it back on at the socket.
The turn the rocker switch on the back of the Psu to on to clear the short circuit error
The psu will not unlock till you do this, as a form of protection/ clear the error stored on the circuits disabled, due to a short circuit no matter how many times you press the power button on the tower.
Stands to reason it`s doing its, job till you fix the short.