It probably got tired and needed a break.  Were you not able to log back in again?  Are you working with a good internet connection?

 Excellent connection. Happened several times. Was able to log in immediately.

Contact Support EITHER https://proconnect.intuit.com/community/proconnect-tax-news-updates/discussion/proconnect-tax-assist... OR https://proconnect.intuit.com/support/en-us/help-article/intuit-account-settings/contact-proconnect-...

OR NEW!!! https://proconnect.intuit.com/community/proconnect-tax-news-updates/discussion/proconnect-tax-assist...

They can probably help you track down the issue. I would use  phone support, but probably not on a Monday.