We'll start by looking at some general troubleshooting tips, then focus on model-specific issues and restart your iPhone. Reset all of your iPhone's settings. A failed update, a jailbreak, or a hardware problem can result in the white screen of death on the iPhone. If that doesn't solve the problem, completely restart (factory reset) your iPhone.
If the hard reset doesn't work either, start your iPhone in DFU (device firmware update) mode. Water-damaged iPhones may or may not turn on. There are also a lot of chances of losing your iPhone data. Even Apple doesn't cover liquid damage in its one-year warranty.