This could be due to any number of reasons. The most common SQL issue is that your host who's using cPanel have had an update installed and the outbound MySQL port (which is disabled by default) has been removed from the firewall, which happens more often than people realise!
However, connection refused is an error message from your MySQL server running on your VPS. Try restarting the service, double check your firewall. No one can give you a definitive answer because the error message is vague.