Every now and again, it seems that even in 2013/2016, the User Profile Service can go south for no particular reason. One error I see most often in the System Application Event Log is Event ID 6398:
If you can't see the image the text is:
The Execute method of job definition Microsoft.Office.Server.UserProfiles.UserProfileImportJob (ID <some guid>) threw an exception.
There was not endpoint listening at http://<server>/ResourceManagementService/MEX that could accept the message....etc.
This error (marked as Critical) fires about every minute.
Turns out that the culprit is usually the Forefront Identity Management service isn't running. To correct, login to the server that is hosting the UPS service then select Administrative Tools > Services (or Start > Run > Services.msc).
Locate the Forefront services and typically, it will be stopped:
Right click on the service and start it - this should clear up the errors and UPS should starting working again.
Make SURE the account running the service is correct - in the above example, the FARM account was used - typically, you would be using an account specific for the synchronization service, right?
The service can be checked in PowerShell and started there as well. To get the status, enter:
To start it: