Hi,
Please try to revert to the sandbox credentials. Does it still work fine ?
If so, it would indicate a problem with the plugin or the live merchant account credentials. In that case, I would recommend you to check with the eway support that your live account is properly activated.
If no, then it would indicate that the problem is actually not with the eway plugin but with something else you've configured between the last time it worked and now. In that case, I would recommend trying to reproduce with the default template of Joomla. This might help in getting an error message displayed, explaining what the problem is.