Sometimes you cant. Although, sometimes all you have to do is take the battery out and let it sit for a little bit. If that doesnt work then it probable is broken. If its water damaged but the phone and the battery seperatly in a bag of rice.
take it to the hospital.
if the screen is black you cant fix it is an internal crack that spread to the external part of the phone.
Call your provider and they send you a new phone just like that :)
If you get a black screen after installing mods, then it is a compatibility or incorrect install problem. If you get a black screen normally, force an update and this will probably fix it.
get a new phone
If the black marks are dead pixels then you have to replace the screen you cant fix it.
only if you have insurence. if you dont then no knew phone.
you can not
Blue screen when playing online
You can fix Wii Black screen without sending it into Nintendo by pulling out the plugs at the back, and re inserting them.
Tell The Kid To Fix IT
The screen itself or the phone itself are the easiest parts to break. Just dropping the phone can damage the screen or the outside of the phone and a cracked screen is hard to fix.