Errors like this are logged. Check the error log on your server. If you can’t find the log, please contact your host.
Meantime, enable wp_debug and wp_debug_log and after an error, look at wp-content/debug.log to see if anything gets logged there. https://codex.wordpress.org/Debugging_in_WordPress
You can also try this: Please attempt to disable all plugins, and use one of the default (Twenty*) themes. If the problem goes away, enable them one by one to identify the source of your troubles.
If you cannot access wp-admin, there are other ways to deactivate plugins.
Thanks for that, but I’m not that technically savvy and wondered if it would be easier to access the site in safe recovery mode to switch off the plugins one at a time. Another post answer said “If your site appears broken and you can’t access your dashboard normally, WordPress now has a special “recovery mode”. This lets you safely login to your dashboard and investigate further.” How do I do this? As far as I’m aware, the admin email is correct, and I’m the only admin.
If you didn’t get an email about recovery mode, I know of no way to access it.
I was given this advice:
“he can use the WP_RECOVERY_MODE_SESSION_ID constant
giving it any value in wp-config.php will force the recovery mode”
At that point, you should be able to login. You’ll need to use FTP or your hosting control panel to edit wp-config.php to add
define( 'WP_RECOVERY_MODE_SESSION_ID', '12345' );
Update: Be sure to remove the constant from wp-config.php once you’re done cleaning up.