From time to time OneDrive can become signed out for security purposes.
OneDrive will show an error icon on your taskbar to notify you of an issue.
Click on the OneDrive icon in the taskbar to open the OneDrive Menu.
Select the Sign-in Button.
In the popup windows enter your Office 365 email and password.
Allow the login process to complete this can take a few minutes on some occasions
Once you have re-signed in your OneDrive will start resyncing your data.