Why is my Samsung Galaxy Grand Prime no longer turning on?

This probably goes back to dropping the phone in water once its dries out it can work fine for some time but over that same time it can form corrosion on the motherboard that will in time short the phone out . you can try taking the phone apart down to the motherboard removing all the shields and protective tape then clean the motherboard in IP alcohol . This guild will help with motherboard removal

Sustitución de la Placa Base del Samsung Galaxy Grand Prime

soak the board for a few hours in the ip alcohol at least 90% + is better then use a fine toothbrush to do additional cleaning on the connectors and contacts. Give any area of the motherboard that you can see corrosion on a good scrubbing and if it still looks bad use a flux compound and a heat gun to give it an extra cleaning. It may also be a good idea to replace the battery . Hope this helps