Clean it with 70% minimum isopropyl alcohol, after letting it dry see if the actual components don't have a burn-in like state (as in the contacts look intact and colour isn't burnt like black) then try turn everything on and go from there.
It could simply be loose wiring when you replaced the phone screen. Open it up and recheck everything is connected properly and it wouldn't hurt to clean the contacts to make sure they dont have residue (from fingers and such) on them.
I would stop turning it on when you've spilt water on it. Your best bet is to open up your Dell following a service manual and using Isopropyl alcohol above 70% to clear up any water marks on the internals of your laptop. By continuously turning it on without clearing all the water of it will definitely damage internal components further and might not be salvageable. If you aren't confident, find a repair technician to clean out the water safely.
i thought the z370 is only compatible with 8th gen Intel CPU's? if it was a z270, they're compatible with 7th gen Intel CPU's. You have a 7th gen Intel CPU (i5-7500) so the new motherboards aren't backwards compatible, everything else will work, you will just need to get a new CPU =), if anything you can use http:www.pcpartpicker.com to help find compatibility in the future.
by cleaning with the isopropyl alcohol i was able to work the laptop back and now have power into it. At first it did not boot up, so I did the whole hold down the power for 10 seconds then plugged in the charger, now i have a working mac!
I've recovered my phone from the same issue. If you really want to know, i had to pull my phone apart and replace the battery since the water got all the way there. If you made the same mistake i did and keep turning it on, most likely you shorted the battery and maybe other parts. I suggest opening the phone and cleaning with isopropyl alcohol (disconnecting the battery and everything first!) any parts that you see have corrosion on it, although check whether you are covered by warranty first to save you the hassle (most places dont cover water damage.) I hope the best for you and you dont have as many issues as mine does now