Messages app stopped working

I've been using AIM since the "AOL days" and I can say without a doubt that the AIM servers have always been unreliable. More so once we started using it via iChat and iChatAV and now Messages.

There are two (maybe more) reasons that I know of for AIM dropping out:

1. The part of their service that checks your login credentials (username and password) goes down.

2. The part of their service that allows clients other than the AOL web client to connect goes down.

I've never been able to figure out where the problem is and at this point there are so many other more reliable ways to connect via Messages that I'm not all that worried about it.

That said, many of my oldest buddies remain on AIM because that was the first system that Apple used with iChat, then iChatAV so it's important to keep that connection alive until we can all migrate to .me or google or something else that Messages supports and is more reliable.

The issue might be the way that you have your notifications for Messages set. If you go under Notifications -> Messages, you will see something that says "Messages alert style:" and will have the option to select None, Banners, or Alerts.

Make sure you don't have "None" set as your alert style. You can also set the badge counter to show for when you get a new message by clicking the "Badge app icon" option lower on the screen.