.mp4 will load in to Premiere, but won't play

Please use the free MediaInfo and post a screenshot of the properties of your media in tree view:
https://mediaarea.net/en/MediaInfo

Many users are having issues with VFR. If the file is variable frame rate, use Handbrake to convert to constant frame rate before importing into Premiere Pro:
https://handbrake.fr/downloads.php
Here is a tutorial:
https://www.youtube.com/watch?time_continue=34&v=xlvxgVREX-Y

Shutter Encoder may also be used to convert to Constant Frame Rate:
https://www.shutterencoder.com/en/

Delete the Media Cache and Media Cache files:
https://community.adobe.com/t5/premiere-pro/faq-how-to-clear-your-media-cache-in-30-seconds-in-premiere-pro-2020/m-p/11017257/thread-id/260446?page=1

If that doesn't work, try resetting the preferences:
https://community.adobe.com/t5/premiere-pro/faq-how-to-reset-trash-preferences-in-premiere-pro/td-p/8236158?page=1

If that doesn't work, try resetting the Workspaces:
Reset a workspace
Reset the current workspace to return to its original, saved layout of panels.
1. Do one of the following:
• Click the Workspace menu icon and select Reset to Saved Layout.
• Choose Window > Workspace > Reset to Saved Layout.
from here:
https://helpx.adobe.com/premiere-pro/using/workspaces.html

If that doesn't work, try Preferences > Audio Hardware and set Input to None.

If that doesn't work, try creating a new project and import the old one into it.