One of my customer phoned me to say that one CentOS 4.x machine (acting as a apache reverse proxy) didn't reboot after a power outage. The machine had two sata disks configured in raid 1 (through md/software raid) but instead of booting, the machine was just displaying a grub> prompt.
Of course i tried the traditional `grub-install --recheck /dev/sda` and `grub-install --recheck /dev/sdb` and also the manual procedure (already described here) to install grub on both devices .. but no luck .. still booting at the grub> prompt.
But then i looked (in rescue mode) at the (/mnt/sysimage)/etc/grub.conf and i counted 22 kernel entries in the file .. The customer had configured the nightly automatic yum update but he never cleaned the old kernels (both up and smp) ... so i "cleaned up" the grub.conf file, once again installed grub with grub-install and .... machine rebooted normally ..
I've never thought that too many entries in the grub.conf file could block the machine from booting ... Maybe that will save other people time