We recently upgraded to vCenter Server 6.0.0 Update 2 in an effort to upgrade our environment to 6.x+ from 5.1.0. After the vCenter upgrade completed fairly smoothly, I noticed one of the 2 hosts we have in the cluster had an alert - "vSphere HA agent is unreachable". There is only one VM currently running on this host and I tried to migrate it to the other host so I could reboot the ESXi host to see if it would fix the issue but got a "general system error" and it failed. I also tried the "Reconfigure for HA" option when you right-click the host in the vSphere Client. In trying to further troubleshoot (check logs, etc.), I turned on SSH and the ESXi Shell but I could not SSH to the host... and when I connect via iLO to view the console of the ESXi host, it does not show the normal Black/yellow VMware ESXI standby screen, only a black screen that has at the top of the screen 'ESXi 5.1.0' (in yellow text) and the VMware URL (in blue text) with the copyright info on the next line. After a while and hitting keys to wake it, it prompted me to login, but I'm not sure what I'm logging into or how to proceed further, or if a restart is necessary (or whether or not it can be rebooted via the client).
How can I fix this so I can move forward with upgrading the hosts to match vCenter? The other host seems to be normal and encountered no issues like this.