You need to configure your video system, what type of video card do you have?
Also to get to a text console (a very important step if you have x problems) you can boot the install cd and start your system in rescue mode, you need to edit /etc/inittab and change the default runlevel so that your not booting to X, look for something like the following,
# These are the default runlevels in Slackware:
# 0 = halt
# 1 = single user mode
# 2 = unused (but configured the same as runlevel 3)
# 3 = multiuser mode (default Slackware runlevel)
# 4 = X11 with KDM/GDM/XDM (session managers)
# 5 = unused (but configured the same as runlevel 3)
# 6 = reboot
# Default runlevel. (Do not set to 0 or 6)
The last line is what you need to change.