Fix WordPress Critical Errors

My WordPress site is suddenly refusing to cooperate: every attempt to upload media, create a new page, or duplicate an existing one ends in a “critical error” screen. When I try to add an image the message reads, “Side.jpg has failed to upload. Could not insert attachment into the database.”
All unnecessary plugins have already been deactivated and removed, yet the problem persists, so the conflict seems deeper—possibly database-related or tied to core WordPress files.

Here’s what I need from you:

• Diagnose the root cause behind the upload failure and the critical-error pages (PHP logs, MySQL tables, wp-config settings, or corrupted core files—whatever it takes).
• Restore normal functionality so I can freely upload media, add new pages, and duplicate pages without errors.
• Provide a concise, plain-English summary of what you fixed or changed so I can avoid the issue in the future.

Access to the admin dashboard, FTP, and phpMyAdmin can be provided as soon as we start. I’d like to get this resolved quickly, so please let me know your estimated turnaround and any specific details you need before diving in.