Replacing a slotted guitar nut?

You can glue it back on, but you'll need to clamp it in place so the nut stays "true" to the neck. Be sure to prepare the site by removing any old adhesive that may be there, and you may need to sand it a little with fine-grit sandpaper -- that'll "rough it up" and make it easier for the glue to stick. Just be careful if you sand it.