I have a Debian Lenny box with two hard drives in it (no dual boot; /home and /var are on the second drive). This machine was working fine until earlier this week when I ran an upgrade and restarted it. I think during this upgrade Grub got upgraded to Grub2, and now when I try to reboot I just get this:

Hitting Enter yields Operating System Not found. This is it, I can't get into the grub screen to enter the boot command or anything.

I booted the machine into the rescue environment on the Install CD, and was able to mount all the file systems and chroot in. I tried reinstalling grub2, but this did not help. I ran fsck on my /boot file system, but it appears fine. The boot partition is also marked with a bootable flag in fdisk. I did try to downgrade back to grub1.5 which, while it didn't given an error did hang shortly after it loaded.

Any ideas? I've seen quite a few threads on this, and I'm beginning to suspect that it's a hardware problem. I've had no trouble reading data off of the disks after I mounted them in the live environment, so I'm not sure. The next course of action I'm considering is to reinstall, but I'd rather check here first.

You can try My Favorite Tool: Super GRUB.
(I always keep one around in case something like this happens.)
At worst, it will allow you to boot your system. You will probably be able to fix it with this tool, as it's the top-notch swiss army knife of booting and GRUB problems.

Use the comprehensive wiki to find answers to your problem(s).

Thanks for the link to the grub floppy, but it didn't seem to help me. All it did was present an option to boot Ubuntu... Perhaps it could have gotten confused? Anyway, selecting this also resulted in an error. I think I am going to try reinstalling, and see if it's a hard drive issue.

That grub image is pretty cool, though. I will be sure to keep it handy.

Another tool you could try is Parted Magic http://partedmagic.com/download.htmlIt can mount partitions and has a terminal for editing files and is self-booting.

Alright, sorry to put this off but I have been a bit busy. I got a chance to play around with the machine, and reinstalling didn't help. (I even gave FreeBSD a shot, but its boot manager seems to be having a similar problem.) I'm guessing it's a bad disk, probably the MBR. This was only the system disk, and I will probably just try to scrounge a new one, though I might give Partedmagic a try.