Periodically Microsoft Office will lose its mind with regard to licensing activation. When it does, you'll see a message like the one below in your Microsoft Applications.
Even if you click "Use Another Account" and attempt to sign in with your known-good Office 365 account, it won't work.
The solution?
Reboot.
Yep. It's inconvenient, but that almost always works. If it doesn't, see these additional articles: