screen won't turn on after battery replacement

There’s a very good chance you damaged the screen in the removal process. I have yet to replace a Pixel 2 battery, but since it’s the same procedure between the Pixel 1 and 2, you almost always damage the screen. I’ve done 2 first gen Pixels since I’ve opened my shop and both times I had to replace the screen. And given the 2 uses an OLED panel, it makes it more likely for the average person or even a skilled tech to damage it. A quick way to test it is to power the phone on and wait a few minutes for it to boot. Although the OLED panel is damaged, the phone should still boot. If you plug it in to the charger and it chimes or vibrates, or if you call it and it rings or vibrates, then we can conclude the phone boots up and the display is damaged.