If you or someone becomes severely dehydrated then the only treatment possible is to go to the hospital and have IV's stuck in you to hydrate your body again.
When you're dehydrated severe enough and go to the hospital you'll be put on intravenous hydration which they'll keep you on for at least 24 hours which by then you should recover enough to be able to drink fluids on your own.
Being severely dehydrated means that your fluids need to be balanced and a lot of fluids need to be replaced.