Summary:
You are trying to access files stored on SharePoint Online from a rich client Office application (like Word, etc). You are prompted to enter you credentials, you do but they are not accepted, you are continually prompted for credentials. This may happen after you changed your Windows password.
Cause:
This is a known software defect.
Resolution:
The click to run version of Office should already be patched to resolve this issue.
The fix for Office 2013 MSI is:
https://support.microsoft.com/en-us/kb/3085480
The fix for Office 2016 MSI is estimated to ship in the July 2016 update for Office 2016.
Check back to this blog post for the KB download link.