Previously when I clicked on the calendar tile the calendar opened with a Microsoft sign-in band across it. Once I signed in, the calendar was there. Up to this point it has been working fine. Today, I went to the calendar & it opened without the sign-in band and most (not all) of my events are gone. I haven't done anything differently (to my knowledge). Anyone have any idea as to what could have happened? Why no sign-in band and where did my events go? I am running Windows 8.

Are you using a Local account to sign-in to the computer?

This issue usually occurs if the app is unable to connect to Microsoft Services and is unable to retreive data which is synced with the Microsoft account.

In case you are using local account to sign in to the computer, I would recommend you to use Microsoft Connected Account.

If you are using Microsoft Connected Account and still facing issues, you can try the troubleshooting steps provided in the article below.
Troubleshoot problems with an app