How to debug in freebsd for all bad condition ?

[Basheer Faith]

Hello

I use Freebsd6. My server sometimes reboots by itself. As the server rebooted I was not near it so I could not see on its display that  what the server wrote on display. 
I want to see what the server gave an error. I could not find any error message in /var/log/message.

How can I see concerning message?
How can I active debug for this case?

Thanks