iMessage on Macbook Pro can't receive or send messages

Hello,

I'm using Yosemite 10.10.1. I used to have my iMessage synced up with my iPhone so that any iMessages I got would go to both and I could use my mac iMessage to text people back if I wasn't around my phone. But about two or three weeks ago, my iMessage stopped being able to send or receive messages. I'm not sure what could have happened or how to fix this issue.

Any help would be greatly appreciated, thank you!