The 500 Internal Server Error is a generic error that can be caused by a number of issues in a WordPress site.
Here are some steps you can take to try to fix the error:
Check your .htaccess file
The .htaccess file is a configuration file that controls how Apache serves files from your WordPress site. If the .htaccess file has been modified or is causing problems, it can result in a 500 Internal Server Error. Try renaming your .htaccess file to something else (e.g. .htaccess_old) and then accessing your site again. If this fixes the error, you can then troubleshoot the .htaccess file to determine the cause of the problem.
Check your plugins
If you have recently installed a new plugin or activated a plugin that you previously had deactivated, it is possible that the plugin is causing the 500 Internal Server Error. Try deactivating all of your plugins and then reactivating them one by one to see if the error persists.
Check your theme
Similar to plugins, if you have recently changed your theme or activated a new theme, it is possible that the theme is causing the 500 Internal Server Error. Try switching to a default theme (such as Twenty Twenty) to see if the error persists.
Check for corrupted core files
If the WordPress core files have become corrupted, it can cause a 500 Internal Server Error. You can try replacing the corrupted files with fresh copies from a fresh WordPress installation.
Increase the PHP memory limit
If the 500 Internal Server Error is being caused by insufficient PHP memory, you may be able to fix the issue by increasing the PHP memory.