After updating a WordPress, or merely doing something minor like attempting to deactivate a plugin, the site might get occasionally get stuck in a non-functional state where it merely outputs an error message like the following:
Fatal error: Call to undefined function nocache_headers() in /wp-admin/admin.php
What might be the cause for this happening?
There were wp files in the web root which had file size of 0 bytes.
Uploading proper copies from the zip file resolved the problem.
The same cause of the problem has occurred to me quite a few times in the past, but I had forgotten it.