Apple Mail not working properly
Apple Mail has a wonderful connection doctor. 

I would open it and see what specific error it finds and then work to solve it from the wizard steps and help that the connection doctor provides for your specific setup and mail provider. 

Once you've done that or if you can't solve it - then you can ask a follow on question or reach out to that mail vendor's support with a very specific error condition and not just "mail doesn't work"


Click the Window menu
Click the Connection Doctor item
Follow the results for any light that's not green