How do I kick the "queue" of messages?

I was sending (immediate, not cron) to a list of about 4800 recipients. I got distracted by an "updated" indicator in the plugins, and with about 450 messages to go, I moved away from the page. Now I have these 450 messages with a "waiting_send" state in the database. How can I force them to send?

Thanks!

Danita