Symptoms were a long delay after entering the address followed by a Square logo that filled the screen.  Scrolling down showed links, including a log in button, but clicking it resulted in error 404 - page not found.  Both Chrome and Firefox were tried, and other computers.  They all gave the same result. I attempted to call, but Square's phone system demanded a customer code.  I didn't have mine written down,  You had to log in to get it.  That was a catch 22.  I searched all my emails from Square and found one with the customer code.  Then I got to a CSR.  She couldn't help me, but while talking, I thought of trying an app on my phone through a different wi-fi.  That worked.  Strangely, as soon as I logged in on my phone, I could log in on anything.  Problem solved, but I don't really have a clue what happened.