So long as all camera doors are closed, the HERO5 Black/Session is waterproof down to 10m (33ft) without a housing.
If the camera is no longer powering on, please try the steps on this article.
Please make sure that the camera is all dried up before proceeding.
Also, see if there are residues on the camera/ battery/ battery compartment.
Let us know how it goes.
You may also reach out to our Support Team for assistance through https://gopro.com/help/ContactUs