/admin not accessible

Hi there,
This morning, there was a notification from defender to upgrade one of my plugins. However, half way, the connection drop off. I tried to access the backend (/admin), however, it shows me error 500.

I check my error log, this is what it shows.

[Mon Oct 10 00:50:49 2016] [warn] [client 115.66.45.163] mod_fcgid: stderr: PHP Fatal error: require_once(): Failed opening required '/var/www/vhosts/***.com/public_html/wp-admin/admin.php' (include_path='.:') in /var/www/vhosts/***-it.com/public_html/wp-admin/index.php on line 10

I tried to search for index.php and it is in the directory. Kindly assist

Thanks.