At the risk of stating the obvious, you can't find a solution until you've found the problem.
The only thing you know is all hash boards are not responding.

You need to shift your focus on what could affect the hash boards, specifically all of them.
4 bad boards is a possibility but unlikely. The hash boards need power and a data connection to the
controller. A problem with either one will make the boards invisible.

I made a couple of troubleshooting suggestions in a previous post, maybe you missed them.