system freezes

[ashas at takas.lt]

I'm using freebsd 6.0.

After some time my system (router+nfs+ftp) freezes. After 10days I 
noticed that I cant ping my system.
The monitor was kept shutdown so I got no message to screen and pushing 
any buttons didnt help out to wake up monitor.
After reboot I haven't fount anything strange in my system or logs.

How to try to find system freeze problem, because it happens at least 
twice a month.

Thanks,
And sorry for my bad English