Our problem has two parts. We tried to send an email to our members and supporters from one user's computer but could not add recipients' addresses (she was away for the upgrade). I tried to send it from my computer instead as I have been doing for the past 6 months. It won't send though - has apparently been 'running' for 2 and a half hours, not being delivered.
As Tomek suggested, try running this task, usually referred to as a Job.
Administer > System Settings > Scheduled Jobs > Send Scheduled Mailings > More > Execute Now
All are not delivered, or part? We faced the same problem, when there was no CRON runnig from server side. If You are sending them via Administrer task, just run this task one more time.