domain.tld is showing 503 Service Temporarily Unavailable error.
At the same time the following error can be found in
[Sat Oct 17 11:50:20.153149 2015] [proxy:error] [pid 12966] AH00959: ap_proxy_connect_backend disabling worker for (127.0.0.1) for 60s [Sat Oct 17 11:50:20.153161 2015] [proxy_fcgi:error] [pid 12966] [client 10.10.10.10:53377] AH01079: failed to make connection to backend: 127.0.0.1, referer: https://domain.tld/
PHP-FPM service is stopped.
Check what version of php-fpm is used by domain in Plesk > Domains > domain.tld > PHP Settings.
Start it, for example for
# /etc/init.d/plesk-php56-fpm start