A lot of people have mentioned their user profile Synchronization service getting stuck on ‘starting’. You could try the below steps to come of the situation.
Firstly, stop the User Profile Synchronization service which was stuck at a status of ‘starting. To do this we will use our big boss PowerShell.
1. Get the GUID of the Profile Synchronization Service application using get-spserviceinstance . It will probably show a status of ‘provisioning’ in PowerShell.
2. Use cmdlet stop-spserviceinstance [Servuce-guid] to stop the Synchronization service.
Now, to troubleshoot the issue lets look at some things
1. After starting the User Profile Synchronization service, wait for 5-10 minutes before proceeding to the next step.
2. Run services.msc and verify that the Forefront Identity Manager Synchronization Service and the Forefront Identity Manager Service are running.These services are started automatically when the User Profile Synchronization service is started. It may take up to 10 minutes for these to start after starting the User Profile Synchronization Service. Do not start them manually. If they are started then stop the service using powershell and proceed to the next step.
2. Confirm that the user account performing this procedure is a member of the Farm Administrators SharePoint group. If you are using a Windows Server 2003 AD DS forest, the Service Administrator account must be a member of the Pre-Windows 2000 Compatible Access group for the domain with which you are synchronizing. Now restart the service.
Lastly, If Central Administration and the User Profile Synchronization service are provisioned on the same server, you will need to run IISReset on that server.