Exchange 2013 is a whole new beast when it comes to a stand alone Exchange server. A failed Exchange 2013 CU (Cumulative Update) can leave your Exchange server unresponsive to outside requests even though all Services are up and Powershell diagnostics are reporting everything working correctly. It leaves many Exchange Admins new to 2013 scratching their heads and attempting to restore from backup only to learn that a restore from backup does not work! What is this sorcery?! Stay calm. This is an easy fix and a lesson for new 2013/2016 Exchange admins that are used to the previous versions.
“Defenders have to be right all the time. Attackers only have to be right once” -Unknown