I had similar situation. We failed over to our secondary because the first one became unresponsive. It wouldn't ping, it wouldnt come up in the admin console, I couldnt SSH to it. Before I rebooted I hooked a monitor up to it. The monitor showed a core dump which I took a picture of. I sent that picture to McAfee and they recommended that I upgrade to the lastest software build (a patch I had to install). They said it was a known issue, and that the latest software should prevent it from happening again. Pulling the power from the firewall and rebooting solved the issue temporarily. So if you run into a situation where you aren't sure what happened to the firewall I recommend hooking a monitor up to it and seeing if there is any errors output to the screen.