How or what can I do to save my phone from being completely dead

Open the Sim tray and inspect the inside. What you're looking for is a bright red mark, you can't miss it.

There is a chance the indicator may not exist, but what it tells you is that water was present inside the unit.

Open the unit to inspect for water damage. Teardown Guide

Notes on how to clean water damaged components

Follow the guides, the screws are critical and the result can be more devastating if placed in the wrong location.

If you don't have the proper tools to complete this task, seek a repair shop in your area.

If the damage isn't from internal water damage what happened/what were you doing before the unit started acting up?