[CentOS] centos 5 reboot diagnosis

Hi we use cent OS 5.2 on all our production server, just today one of or mail server had a reboot and we dont understand why ... usually we get a alert if a system was loaded up ... but this system just reboot .... may be it could be a power failure ...( is there any way to check this ) .. but that is the last optiopn i am guess as the server is in IDC . With dmidecode and biosdecode .. i just get server / bios info . In the Messages and Dmesgs ... i cant really pin pont the issue . Is there any thingi am not doing or any thing i am supposed to figure out the reson for this reboot.