Please deactivate all the 3rd party plugins and activate only theme-required plugins on the site and then check the issue. I am sure your issue will be solved. Then Activate the 3rd party plugins one by one and check which plugin is creating the issue for you.
Otherwise, if the issue still exists then keep the 3rd party plugins deactivated and share the wp logins details so i will check and give you a possible solution.
Tried deactivating the plugins, but the issue is not resolved. Please check the site and fix it or let me know ASAP. But the details page is showing 500/503 internal server error.