Why after replacing screen is the phone black after powering on

Being a Pixel owner myself as well as a volunteer moderator here on iFixit, Pixel issues naturally tend to catch my eye. I've noted a disturbing trend with Google's genuine replacement parts in that it seems to me there's a higher rate of defective screens than there really should be.

My point here is that if this is a genuine Google replacement part, then I would definitely suspect that the new screen may be defective, not to discount that same possibility even if it's an aftermarket screen. Assuming the old screen was at least functional enough to show an image, then I'd suggest plugging it back in to see if it still works. If so, you can be fairly confident the new screen is bad and you should contact the vendor for a replacement. I don't know where you got yours, but as an example iFixit offers a lifetime warranty and would easily replace it for you.