Possibly you updated the kernel-* packages, and you need to create a new initrd containing kernel modules for the same version of the kernel that you are booting from.
This should make you able to boot from the installation media:
https://docs.slackware.com/howtos:sl...oot_from_media
Once you are running, check the contents of the /boot directory for the installed kernel version.
Then, ensure a proper /boot/initrd.gz exists, or regenerate it using mkinitrd:
https://docs.slackware.com/howtos:sl...ystemupgrade?s[]=mkinitrd (look for mkinitrd about at the middle of the page)
Then, depending on the bootloader you want to use (lilo, elilo, grub, ...) you need to inform it of the correct location of your kernel and initrd files. At the same site you can find documentation about the different bootloaders.