iMessage isn't working on my mac?

It was working fine before, I was sending and revcieving messages. Now I can send a message and it will show up. Say I minimize or close the window and someone will reply or send me message, I get the little badge notifcation in my dock icon and I see a little banner type message in the side with a small bit of the conversation like "Hey how are y..." and I can only view it from there. When I open my iMessage to see what they said its not there.