How do I get my old Prismacolor pencils to work again?

Old Prismacolors that have dried out are pretty much a lost cause - the wax binder just hardens over time and there's no real way to reverse that. You could try sharpening them really well and testing on scrap paper to see if any are still usable, but if they're not laying down color smoothly they're probably done. I'd say toss 'em and maybe grab a fresh set if you're getting back into coloring, though you could always donate them to a school or art program first if you feel bad about it.