Hi,
Hard to answer wqith this little information.
Please go to Acymailing->Configuration->Queue process and check your last cron time. It should matchd your cron interval. If the last trigger is a long time ago your cron is not working. In that case check if you have a valid Essential or Enterprise subscription. Without it your cron will not work.
If you have a valid subscription go to Acymailing->Configuration->License. Unlink your license, click save, link your license again, click save and activate the automatic sending again. Then go to Acymailing-Configuration->Queue process and make sure you have enabled automatic sending.