There is no one single reason why a cellphone screen goes black. Sometimes it can be caused by the device's software, while other times it can be a hardware failure in the actual LCD component. The process for getting your screen back to life depends on figuring out why the screen is black, along with making a few other considerations surrounding your phone.
The simplest of all the cellphone screen issues to correct is a minor software glitch. These glitches can come out of nowhere and do many strange things to your device. To correct the problem, simply remove the cellphone's battery, wait a few seconds, and then put the battery back in. Power up the device to see if the quick reset corrected the screen issue. Additionally, some devices also have a hard reset that you can do without needing to see the screen. If your device has this kind of hard reset function, and you don't mind losing all of the information in the cellphone, a hard reset can sometimes correct issues that a soft reset will not.
The most common reason for a cellphone's screen to go black is a simple hardware failure. This can be caused by the actual LCD going bad, by the cable that runs between the LCD and control board going bad, or even just from the cable's connectors coming loose. To fix these problems you need to open up the cell phone and replace or reseat the defective part. The exact process and tools for taking your device apart will vary wildly by model, but most just require a small screwdriver and a simple pry tool. Once the device is open, inspect the LCD and cable for damage; if none is found, simply replace both parts.
When water comes into contact with an electrified board, the impurities in the water can allow current to flow in directions that it was not intended to, causing a short circuit. Even if your control board isn't damaged by a short circuit, the corrosion caused by the water can impede the flow of electricity and cause certain areas of the cellphone to no longer function. If you find green or brown areas of water damage on the circuitry, cleaning it off can sometimes get your device working again. Use 98 percent isopropyl alcohol and a soft brush to remove the corrosion, then put the device back together to see if anything changed.
If your cellphone is still under warranty, then your best option may be to take it back into the store where you purchased it. Some stores have qualified technicians available to repair your device, while others will simply replace it with another phone. Either way, your cellphone should be working again. Note, however, that this is not an option if you have opened your device to try and repair the screen or if your cellphone has water damage, as these both will void the warranty.
Instead of a warranty repair or replacement, if you have purchased insurance on your cellphone you can also choose to contact the insurance company. You'll have to pay a deductible, but the insurance company won't care if you have opened or damaged your device.