Sporadic electrical issues can be the worst to diagnose.
Turn it on and measure voltage going into the panel, make sure you're getting power there.
If so, check for signal voltage at the relay/ssr. If it's a physical relay can you hear it clicking?
My guess is that part is bad. I've only seen temp controllers go if they get super hot, and as long
as the panel connections are good, that only leaves you with the relay.