My Motorola Moto G 2nd Gen fell in water.
Unfortunately, as with most unprotected electronic devices, water does unspeakable things to cell phones. It's for this reason that diagnosing challenges with water-logged phones is time consuming and unreliable.
Water can short circuit the device in the short run, water also leaves behind moisture. This moisture causes rust and calcification of any contacts and other exposed metals inside the device. It is entirely possible that this process has caused your screen to lose connection/power to the device it's attached to.
Best case scenario: Take it back to the service centre and see if they will fix it again (whether or not they'll ask for money this time is up to them). It could require a new ribbon cable or other quick part to get it up and running again.
Worst case scenario: It's not repairable, and if it is, you may continue to encounter challenges like this until the device ultimately fails completely.