Firstly, you posted this in the wrong sub-forum. This one is for vRealize Automation. You should move this to vRealize Operations. Secondly, you have a really old version of vROps that is in dire need of upgrading. Third, how are you monitoring this DC? It's possible it rebooted so quickly as to fall under vROps' default 5-minute collection window.
Thanks, I have moved and should have be more specific in my selection. Yes understand it is an old version and am looking to upgrade soon. Yes I did wonder whether it fell under the threshold to alert but could not find settings, and if the default could be modified