24 : Attempt to access block outside partition
This error is returned if a linear block address is outside of the disk partition. This generally happens because of a corrupt filesystem on the disk or a bug in the code handling it in GRUB (it's a great debugging tool).
To vypada na chybu disku nebo filesystemu. Zkuste nabootovat do LiveCD, pripojte oddil s nainstalovanym Ubuntu a pustte na to fsck.