Hi all ( @TylerT and @HannesZiegler ) - I just had (breifly) and resolved the same issue. Hopefully this can help you Tyler. As Hannes said though, you may not have a config file. I may have created one in the past - I do not recall. BUT, I do have one now and changing the path to our new network root in the config file fixed the issue.
# c.NotebookApp.notebook_dir = r"c:\path\old_network_drive\to\dir"
c.NotebookApp.notebook_dir = r"c:\path\new_network_drive\to\dir"
In my case, I absolutely know what happened. We have a cloud-based server through Box (that's a company). Our IT just changed the base directory name, and immediately Notebooks failed to load. My troubleshooting was initially like yours - I copied my Notebook file (.ipynb) to my hard drive and tried loading it in Pro from there. Same result - wouldn't open.
Perhaps try one more time to discover your jupyter_notebook_config.py file by opening the command line and entering command: jupyter --paths - this should list a few directories. Mine was in c:\Users\myprofile\.jupyter. The config file was located there.
Also note that I recently "upgraded" from 2.9 to 3.02 and did NOT encounter this issue.
Hope that helps.
Zach