Azure AD Connect not syncing to Office 365

Recently we received a call from a client advising that he was in the Office 365 Admin Portal, and noticed that there hadn't been a sync for the last 3 days or so.  He'd asked us to log in and take a look.

DirSync, or Azure AD Connect as it is now called, relies on two account credentials to work properly.  A domain admin account, and an admin O365 account.  Generally, you will create a service account for Azure AD Connect, mark it as a Domain Admin, and then never change the password again.  The Admin Account you should rarely change the password to either.

In this case, the password for the Office 365 Admin account had been reset, which caused authentication to fail and the sync to no longer work.  Running the Azure AD Connect wizard again to re-enter the new credentials will resolve this issue straight away, and you will notice the synchronisation happening immediately.