Why won't my iPhone turn on after getting wet?
Usually because liquid has bridged connections on the board or corroded the power circuitry, so the phone cannot complete a boot. Occasionally the phone is running fine and only the display has failed. Both are common, and neither can be told apart from the outside — the board has to be opened and cleaned.
What is probably happening inside
Water carrying salts and minerals makes paths where there should not be any. The phone may draw current and get warm without booting, may vibrate but stay black, or may charge for a moment then stop. Corrosion also attacks the tiny solder joints around the power and charging circuits, which is why a phone can die a day or two after the soaking rather than immediately.
A liquid-detected warning at the charging port is a separate thing: it means moisture is still sitting in the connector. Do not override it and force a charge.
What to do now
Leave it switched off and unplugged. Do not keep trying the power button — each attempt pushes current through wet circuitry. Bring it in for an ultrasonic clean and corrosion treatment, charged from €99 as a non-refundable deposit, and you will get a clear answer about what survived.
If it does come back, back it up immediately. A board that has been wet is always more fragile than one that has not.
The signs worth checking before you give up
Connect it to a computer. If the computer sees the phone but the screen stays black, the phone may be alive with a dead display — a much cheaper repair than a dead board, and your data is still reachable.
Listen and feel for signs of life: vibration when you toggle the mute switch, sounds when a call comes in, warmth when connected to a charger. Any of those means the board is doing something, which is a better starting point than total silence.
None of these checks involve opening the phone or forcing it to boot, so they are safe to try. What they cannot do is tell you how far the corrosion has spread — that only becomes clear once the board is out and cleaned.