The brightness of my Samsung j7 star is not working

@giovanni6785 seems like what you have here is a water damaged phone. For now, you want to stop continuing to try to charge, sync or otherwise use your phone. This potentially can make a bad situation worse. This may further damage your phone.

The next thing to do is to clean it. Disassemble your phone using [these guides] . You must remove all EMI shields. If you do not remove those, you might as well not do anything. After you removed those, clean the whole board, connectors and all with +90% isopropyl alcohol. Follow this guide , and even so it was written for an Apple iPhone 3G, all the points are still pertinent to your phone. While you clean your board, check for any obvious damage, like burned or missing components etc. When it is properly cleaned, replace the battery. All these steps are to avoid delayed failure caused by corrosion. Best thing to do, would be to clean it with an ultrasonic cleaner, but if you do not have access to one, this will at least help. Once all this is done, reassemble your phone and reevaluate. Until it is cleaned, everything will only be a guess. It is possible that you have a shortened LCD/backlight and replacing the display assembly may fix it. Even with all this, water damage has only a small chance to be fully repaired, but at least you can give it a try.

The backlights of the LCD are fried, you can't replace only the backlights, you need to replace the whole LCD.