I am not sure why, while upgrading, installing and uninstalling some stuff my system broke, so that on booting, just after the boot line selection in the grub(2) menu, the error “command not found: initrd” (or similar) appeared.

You can fix the grub installation again using any kind of live CD. I used the Knoppix CD.

• Boot the live CD
• Mount your boot partition, for example to /mnt. In my case

  mount -t auto /dev/sda1 /mnt

• Bind your /dev directory to the mounted partition:

  mount -o bind /dev/ /mnt/dev

• Chroot into your boot partition

  chroot /mnt

• Mount your proc directory

  mount -t proc proc /proc

• Mount your sys file system (/sys)

  mount -t sysfs sysfs /sys

• Ready, now you can use your system almost as if you booted in it. You can reinstall grub, kernel images, create initrd images, fix the menu.lst or grub.cfg or whatever you want.

And, by the way, if grub is giving you something like

error: the symbol grub_machine_fini was not found

on booting, it is installed to the wrong hard disk (or not installed). Install it to the master boot record of the hard disk (not to the partition!) In my case:

grub-install /dev/sda

(and not sda1)