Having come across this error enough times (and tired of searching for it) - occasionally, you might see an error regarding the SharePoint 2013 User Profile Synchronization service. Noted as a Critical Error in the System Application Event log, it shows something like this:
The Execute method of job definition Microsoft.Office.Server.UserProfiles.UserProfileImportJob (ID f8a883d5-a63e-44b9-9ed8-90cdceebc96c) threw an exception. More information is included below.
There was no endpoint listening at http://sp13:5725/ResourceManagementService/MEX that could accept the message. This is often caused by an incorrect address or SOAP action. See InnerException, if present, for more details.
Notice that SP13:5725 (my server) will be different on yours.
As it turns out, this is usually caused by reboot in which the Forefront Identity Management (FIM) service has stopped (or didn't start). Using Administrative Tools > Services, scroll down to the FIM service and indeed you will see it will not be running:
Simple click the "Start the service" link to start it up. On next synchronization attempt, all should be good and the error should go away.