NT 4.0 server rebooting automatically

Has anyone seen or had this problem before. We run Small Business Server 4.5 on NT 4.0. One day out of the blue we rebooted the server and when it gets to the point (blue screen.. SP info) reading the partitions the system automatically reboots, We cannot boot the server because of this, not in VGA mode or normal. We have tried the HD in multiple systems with the same problem occurring. Hard Drive diagnostics do not show any errors.