website running php-fpm shows 503 Service Temporarily Unavailable

Website is showing the following error:

503 Service Temporarily Unavailable

The following errors are shown in /var/log/plesk/panel.log log file:

[proxy:error] [pid 12966] AH00959: ap_proxy_connect_backend disabling worker for (127.0.0.1) for 60s
[proxy_fcgi:error] [pid 12966] [client 203.0.113.2:53377] AH01079: failed to make connection to backend: 127.0.0.1, referer: https://example.com/

The website is using PHP-FPM handler at Domains > example.com > PHP Settings

The following errors can be found in the website error_log file at /var/www/vhosts/example.com/logs/error_log:

[proxy:error] [pid 3838:tid 140126940247808] (2)No such file or directory: AH02454: FCGI: attempt to connect to Unix domain socket /var/www/vhosts/system/example.com/php-fpm.sock (*) failed
[proxy_fcgi:error] [pid 3838:tid 140126940247808] [client 203.0.113.2:56904] AH01079: failed to make connection to backend: httpd-UDS

1. Connect to the server via SSH
2. Get the PHP-FPM version assigned to the domain:

plesk db “SELECT d.name,h.php_handler_id FROM domains d JOIN hosting h on h.dom_id=d.id WHERE d.name=’example.com'”
+————-+—————-+
| name | php_handler_id |
+————-+—————-+
| example.com | plesk-php73-fpm |
+————-+—————-+

Start the PHP-FPM service with the required version. For example the Plesk PHP-FPM 7.3 service:

service plesk-php73-fpm restart