If you have the tools and think you know how to take it apart, try taking it apart by following iFixit’s guide found here: Apple Watch Series 4 バッテリーの交換
Apple Watch Series 4 スクリーンの交換
(These guides are the furthest iFixit will tear down the watch, the tear down does disassemble it further, but it doesn’t show precise instructions)
After this, try cleaning off any corrosion you see with a clean toothbrush and isopropyl alcohol. If you don’t see any corrosion, lucky you. Mine had lots of corrosion everywhere.
While you’re in there you can also replace the battery if you want and make sure to replace the screen adhesive, also you might want to fix the screen as well. Keep in mind that the watch likely won’t ever be waterproof again, even with a new screen and adhesive, so be careful not to expose it to water.
If I were you I would take it apart to try to clean it, since you have nothing to lose if the screen is cracked and it won’t turn on. You will need a hobby knife (about $10) and a tripoint Y000 screwdriver (Tri-point Y000 Screwdriver
Buy a high quality tripoint screw driver, the cheap ones strip very easily.
Also, don’t worry about destroying the Force Touch sensor. In WatchOS 7 it’s completely disabled anyway.
Edit: changed screwdriver to correct size