You will notice that the AzureAD Sync tool stopped synchronizing and in the Office 365 portal under Health Directory Sync Status you will notice the error message Warning: no recent synchronization  under Password Sync.

In Powershell when you run the Start-ADSyncSyncCycle you will get the below error

Warning: no recent synchronization 

Start-ADSyncSyncCycle : System.InvalidOperationException: Scheduler is already
suspended via global parameters.


To fix this, simply open Powershell and run the below command.

Set-ADSyncScheduler -SchedulerSuspended $false

After it completes, re-run Start-ADSyncSyncCycle and it will work.