Remove the case and screen protector then charge the battery until it's 100% then restart your iPhone.
How to restart[*]Press and hold the Sleep/Wake button until the red slider appears.[*]Drag the slider to turn your device completely off.[*]After the device turns off, press and hold the Sleep/Wake button again until you see the Apple logo.
If that doesn't help, make an appointment at a Genius Bar
Liquid damage to iPhone or iPod isn't covered by warranty - Apple Support checkcoverage.apple.com
Find the serial number of your Apple hardware product - Apple Support