Did you make a boot disk during the install?
If you did - put that in the floppy drive and then boot the machine.
You might even be able to use the Slackware install CD as a recovery CD - I know that you can with lots of linux distros. If you can it will give you the option at the CD-boot screen (where you're ment to specify the kind of install that you want.)
As soon as you get into the system, reconfigure Grub so that it works - just sounds like the right drive to be booted hasn't been specified to the grub loader.