Why is my screen black and not lighting up

Unfortunately, most phones are only water resistant and not actually waterproof. Once exposed to water, generally the LCD is the most easily damaged part, so it's highly likely that the screen is ruined and will need to be replaced.

Generally liquid damage needs to be addressed immediately. Shut off the phone and do not attempt to charge it until you've cleaned the phone and removed all traces of liquid. Here's a guide to handling a phone that's been exposed; it was designed for iPhones but the principles are applicable to all models of phones.

iPhone Liquid Damage Repair - iFixit Repair Guide

You can try drying out the screen; remove it from the phone and put it somewhere warm but not hot overnight; it's possible that it could work again once the liquid has dried, but you should probably be resigned to replacing it. A new battery is advisable too.

You'll need to provide the model number of your phone if you want more help; if we know which phone you have we can probably find guides to show you how to disassemble it for cleaning and replace the screen and battery.