It sounds like your host is having some MySQL issues OR you have two copies of the same mailing running at the same time. To be sure your previous mailing has stopped, close the window and re-login to ListMail. You will see an item 'in queue' at the top of all ListMail pages. Browse around the program and you will see the number of messages remaining decreasing. Once they stop decreasing for about a minute, it's safe to Resume. I will be implementing strict checking for active queues very soon as this seems to be a problem on a number of hosts.
Regards