I'm guessing you jumped the CMOS? if not start there also as others have said, check your memory is seated properly, you'll be surprised how firm you have to push down...I'm sure you already know this but push down on the memory until the little retaining clips on each end of the slot snap into place, if not then it's not seated properly and you could have got lucky the first time around and made a brief connection same with graphics card. also try another HDMI or display port cable, rearranged my setup last night and the monitor wouldn't come on and it was the display post cable that was at fault. Did you set the XMP memory setting in the bios, that thing has screwed me so many times and it wont boot, uncheck this if you did.
If you can post to bios ( i'm guessing you can't) check the boot order of your storage devices. If you installed windows using a thumb drive then remove it and try again without it, I've had a problem there once before.
Again this is obvious but check the cpu fan header and that you have it plugged into the right header on the mobo, I messed this one up last week when I installed my new Ryzen 3900x, I put the fan cable into the water block pump header on the mobo by mistake, is your cpu fan even spinning or water pump working?. Another obvious one, did you try the cmos reset? if so was it via a button on the mobo or did you have to jump the pins on the mobo and forgot to move them back or missed and went too far and only one pin is now jumped.
Also, as someone else said, is your memory in the correct slot? you would think that the slot closest to the cpu would be the correct one but on my board it's the second one in (slot 2) and slot 4 to use 2 sticks for dual channel.