Resolve: If in a single management server environment, take the management server out of maintenance mode.
Never put a management server in maintenance mode: https://technet.microsoft.com/en-us/library/hh212718(v=sc.12).aspx
I am new to SCOM and recently started using ‘maintenance modes’ based on the following article:
My logic while flawed was to put the server in maintenance mode after applying updates and reboot. I left it in maintenance mode to resolve a couple of start up errors but all monitoring ceased. Take the server out of maintenance mode…and all was well…humbling… easy fix.