Background story (that you can safely ignore)
If you have setup a policy to enable Load Evaluator Index in Citrix, you will be able to check if a machine is at a bottleneck and require more resources to be able to perform better. By default, Load Evaluator Index is not used. We will not talk about how to enable it in this post. Once it is enabled, they use a formula to calculate the Load Evaluator Index by looking at the CPU / RAM / Disk usage and the number of active sessions on that specific machine. However, I have seen cases that when I RDP to the VDA, the CPU and RAM are at the lowest possible and there is 0 session connected, but the Load Evaluator Index is still showing 100% and shutdown the agent to disallow new connection to connect to this VDA.
To troubleshoot this issue, RDP to the VDA in question. Try to launch a Perfmon, if there is an issue with some of the counters, it will display a message similar to this one.
This indicates that the counters are corrupted. To rebuild the counters, launches CMD as an Administrator. Most likely, you are using a 64 bits system, you will have to go to c:\windows\syswow64, otherwise c:\windows\system32 is the path for 32 bits systems.
type in LODCTR /r to rebuild the performance counters from the system backup store.
When that is finished, go back to the VDA and restart the Citrix Desktop Services. After a few second, the Load Evaluator Index shown on Director will refresh and the correct Index will display, allowing new connection to connect to this VDA.