Emergency recovery

You may need to recover the system if the files in /boot are damaged. Specifically, the /boot filesystem is vfat, and therefore can easily be corrupted if you power off the system without a proper shutdown. Using a Slackware host system with an SD card writer, for example a PC, you can restore these files from a backup, or you can reinstall the boot and kernel packages something like this (replacing sdb with your SD card device name, and /path/to with the path to the Raspberry Pi system packages):

e2fsck -p /dev/sdb3
mount -t ext4 /dev/sdb3 /mnt
dosfsck -a /dev/sdb1
mount -t vfat /dev/sdb1 /mnt/boot
ROOT=/mnt upgradepkg --reinstall /path/to/raspi-boot-*.tgz /path/to/kernel_raspi-*.tgz

Safe mode

An alternative is to use the Pi's "safe mode" by adding a jumper across pins 5 and 6 of P1 and booting. The Pi will then load an alternative (non-Slackware) emergency kernel with a Busybox shell, but only if most of the boot files are still good. See this forum post for details.