If you’re on this post right now some of your Citrix VDAs are showing Power State Unknown. You may be getting help desk calls from people saying they can’t launch anymore. Try not to panic!
When a VDA shows Power State Unknown, even if it’s out of Maintenance Mode, it will not take new sessions. If you find yourself in a situation where you have Power State Unknown on most or all of your VDAs, that can result in big problems as users won’t be able to launch new sessions and the few VDAs in Power State On may crash due to a user overload.
As you’ll see in the Citrix docs, this happens where there is an issue between the Delivery Controllers and your hypervisor of choice. An example of when this might happen (as I have experienced) is when firmware upgrades are taking place on the hosts used for the VDAs. It can happen when a machine identifier in the hypervisor changes and causes a mismatch with the identifier in the Citrix Database. The DDC knows there’s a mismatch, assumes there’s a problem and takes protective measures by not allowing sessions to this questionable VDA. It can also happen when there is a momentary service or connectivity issue between the DDCs and hypervisor.
The real fix is figuring out what caused the issue and if it was unplanned, ensure it doesn’t happen again but if you need to start getting your VDAs back in working order you can either restart the Broker Service on your DDCs one by one (leaving a few minutes between each) or alternatively reboot the DDCs.
You could also remove VDAs from your Delivery Group and re-add them but this is less than ideal. I suggest the other two methods.