First, just to cover all the bases:
ctrl-alt-Fn means hold down the "Ctrl" and "Alt" keys simultaneously and then press F1,2,3,4,5, or F6 once. (1-6 is usual, but it could be different.)
Don't get offended if you knew that (and I suspect you did). I guarantee, some people don't.
Also, double-check your capslock, numlock, etc.
The next thing to check is /etc/inittab
It should have several lines that look something like this:
(mingetty is the program on my Red Hat 9 system. There are a dozen different flavors of "getty". Any of them should be ok, so don't be too surprised if you see "getty", "agetty" or something like that.)
If these are missing, you won't have any virtual consoles. If they're not set to "respawn", you could use each one once, and then it would be gone forever.
I haven't run Slackware in quite a while, but I'd be very surprised if these lines were missing.
Next, run "ps -ef | grep getty". If you have several of them running, and capslock and so forth are off, I'm stumped. You might want to bounce your X server, in case it's gone slightly psycho. Beyond that, I'm out of ideas.
If you don't
have any gettys running, and inittab does say "respawn" (which it's almost certain to do), then there's something wrong with your getty. Run (at a command line) "/sbin/mingetty tty4" and look for error messages. If you don't get any, try Ctrl-Alt-F4 and see if it lets you in.
One last thought -- have you loaded any unusual keymaps? It would be a very
unusual one that change the Ctrl-Alt-Fn keys, but it's the only other answer I can think of.