An increasing number of Android users who still run phones from Google's 2018 flagship Pixel 3 series have reached out for help online after their aging yet still (heretofore) trusty devices abruptly stop working, often "overnight". The handsets in question reportedly develop the same potentially fatal issue, in which their screens turn black and fail to react to any combinations of button presses.
Issues such as these can often be resolved. However, in this case, there appears to be no consistent fix to date. Some users involved in this phenomenon assert that it occurs because their 3-series unit spontaneously enter a "Qualcomm Emergency Download" (or EDL) mode, in which the phone seems to shut off abruptly and cannot be convinced to turn back on in any way.
This EDL mode is linked to phones that show up as a QUSB_BULK_CID device on connection to a PC. The problem is thought to be related to either a firmware update or to simple hardware failure over time at present. Google has yet to respond with a solution, although the Mountain View giant has at least opened an Issue Tracker for the complaint.
This page shows that the problem can affect either the Pixel 3 or 3 XL. However, as both devices are out of warranty, or will be soon, a conclusive fix may not be in the offing for their owners.