Wordpress Site (On Digital Ocean) - Needs Fixing Urgently

I have a wordpress website hosted on Digital Ocean.

Today, somebody tried to install a development server on the Digital Ocean droplet (to hold a duplicate copy of the site) but they have caused the site to go down with the error page shown below:

I need to have this fixed and the site back up ASAP.

503 Service Unavailable
PHP did not respond properly to the request. As a result, Apache considers the PHP-FPM service to be unavailable.
Detailed information about this error is available in the app's Apache error log at:
/srv/users/SYSUSER/log/APPNAME/APPNAME_apache.error.log
The root cause of this error is often slow PHP code. You should disable any plugins the app is using and check the app's PHP error log and PHP slow request log located at:
/srv/users/SYSUSER/log/APPNAME/APPNAME_phpX.Y.error.log
/srv/users/SYSUSER/log/APPNAME/APPNAME_phpX.Y.slow.log