First off, make sure your internet connection is ok, and that you're using the correct login and password - sometimes it's the simple answer!
If that's all looking good, we may need to do some deeper trouble-shooting for you. You should see a little speech bubble icon in the lower right-hand corner of your screen. Click on there and tell us what's happening including: Any error messages you're seeing, the name of the browser you're using (chrome, safari, internet explorer etc), and what kind of device you're on.
We'll try and re-create the problem at our end so we can see what's happening!