Please help! Water damage

Take it apart as best you can without breaking it, and try to dry all the parts. Dry it in parts for 2-3 days before putting it back together and trying again. I hope leaving it plugged in overnight with the water in there didn't fry it

When he says 2-3 days, he's not kidding.

Prematurely trying it will definitely fry it.

Maybe you could stick the laptop or the parts in some uncooked rice, to draw the water out?

A little tip would be to put it in a bag with dry rice the rice will help absorb moisture. Hopefully you didn't fry anything. Keep it without power for a few days after cleaning it good may come back to life.

The rice trick generally doesn't help, but if you really want, you can try it (it certainly wouldn't hurt anything).

Because the battery was connected for so long, the chances of recovery without repair are very slim.

However, the best bet would be leaving it for 48 hours without anything plugged in (and the battery removed). If there's a chance, that'll be your chance.

It's generally not a good idea to take the computer apart yourself. It's easy to inadvertently cause damage through electrostatic discharge, and if parts are already severely damaged, taking the computer apart or cleaning them isn't going to fix them.

If, after 48 hours, it still won't turn on, your computer needs to be repaired by a Mac specialist. You can call some Apple service providers who are local to you, but Apple's pricing is outrageous ($755 or $1240, depending on the computer). Most shops can't do cost-effective liquid damage repair (it's very specialized), so you may need to use a mail-in service.

Putting in in an airing cupboard would be a good place ta put it while its drying.

By the way, let us know what happens when you try to turn it on.