If you can get to the (normally hidden) grub menu, you might find that there's a backup kernel you can boot from. Try hitting tab intermittently between the screen clearing after the initial bios messages & getting the error 18. You'll hopefully be presented by a menu that lists the kernels you can boot from, with the default one being highlighted. If it shows more than one, use the cursor keys to select the another one & hit return or "b". It should boot normally if there's nothing else wrong.
If that doesn't work then as others have suggested a live USB key rescue image would be the next step. Be aware though that if the filesystem is a bit corrupt & you run an "fsck" (filesystem check) on it you might do more damage that someone else could recover.
That grub error 18 to me suggests that there's some filesystem corruption. As you get as far as that error, the grub bootloader is ok, the filesystem is partially ok as it can access the grub.conf configuration file, but there's an issue with the disk address the kernel is at.