This can happen if your browser has some bad data in the cache or cookies.
Clear your browser cache. This data sticks with the browser even if you restart, so you need to go into the browser settings/options and clear the cache. Some browsers and systems require more frequent cleaning than others. Then close and restart your browser.
If that doesn't solve it, try using a private or incognito mode window. This will hide the cookies that are in the memory. If you can log in and play through the private/incognito window then you will need to clear your "cookies and other browsing data" again in your browser settings/options. Then it should work normally.
A different browser sometimes will have better results as well. Chrome is the recommended browser for the best gaming experience.
If you still need further assistance please use the link in my signature to fill out a support ticket.
