Fatal error – media.php file has been changed

Maybe you are just out of date. There could be several reasons.

Make sure the WordPress core, plugins and themes are all up to date.
Replacing just one file doesn’t really make sense.
https://www.hostinger.com/tutorials/how-to-reinstall-wordpress

You can scan you site for malware :
https://www.ads-software.com/plugins/malcare-security/
https://www.ads-software.com/plugins/wp-malware-removal/

If you still get errors, then disable your plugins, and add them back in one-by-one to see if you can isolate the issue.

Thank you for this prompt reply. I have tried reinstalling and updating WP and all plugins. Selectively disabling plugins has proved difficult, because the site runs perfectly for some time and then crashes. I know that replacing one file doesn’t make sense but that is the fix I’ve found … very strange.

Thank you for the malware scanning plugins. I’ve installed and I’m running it now.

Login to your host, and read the php and apache error logs.
Also, check your version of PHP on your host.

You can run this as well :
https://en-ca.www.ads-software.com/plugins/health-check/

Malcare found some issues and cleaned them up. I’ll see if this does the trick. From previous experience everything seems to be working correctly … and then several days later the Fatal Error reappears. Hopefully this malware scan and clean has solved the problem.
Thank you for your advice.