Q: Messages App on Macbook stopped working

I've been using the Messages app forever now, and it has suddenly stopped working. I can sign into my gmail account via Messages, but not my iMessage account which is what I want to do. I am fully connected to the Internet, but when I enter my Apple ID info (which is correct, I've triple-checked), this is the screen it shows me. I have tried many of the tricks I've found on here, and am at a total loss. This is trickier since the computer is my company's, and not mine. So I may not be able to follow tips if they are too involved and would need an admin's help. Thanks.