OK - I contacted eWay and eventually they asked what PHP version I was using (I checked in zencart under Tools and it was 5.2.17). The eWay module I am using requires PHP 5.3. So that was the reason for the fail. It turns out that my host Hostgator did some moving around of sites a while back and must have downgraded the PHP version. I can't really blame them because they did advise me that things were going on and asked me to check my sites for problems. Trouble was, I was just about to leave for overseas and didn't have time to do anything. So, mystery solved!