Hi,
Looks like you have a problem in the config of your send process. First, check if you have a notification in Acymailing's message center. You will find that in the top right corner of Acymailing.
Next thing you can do is enable debug mode and set error reporting to maximum in your Joomla Global settings. Maybe that will give you some additional information.
Let us know and please inform us how you have configured your send process.