Harry's probably on the right track here, but let me add to this an experience I had with my niece's Pixel 4 XL. Similar to yours, it appeared 100% d-e-a-d. Here's the thread where we discussed it.
[post|740760|Diagnose Pixel 4 XL not booting - Google Pixel 4 XL - iFixit]
The TL;DR version is the only thing I did to this phone was unplug the battery for a half hour; when I plugged it back in, it started working again like nothing was ever wrong with it.
My suggestion here if it was mine is I would go ahead and buy a new battery and swap it out. Just leave it unplugged for a good half hour total, then plug the display back in and see if it works.
[guide|136194|Google Pixel 4 Battery Replacement - iFixit Repair Guide]
Let us know what happens; if the battery replacement doesn't do it we'll try and figure out what your next step should be.