Why won't my LG VS840 Phone power up?

When I plug my phone in to charge...as soon as I plug it in, screen lights up first with LG logo then shows my battery is fully charged. Then screen goes black. When I try to power up by depressing power button, it will not come on. I've tried to take battery out then back in and same with my sim card. I've tried double clicking the power button, long depressing power button and just a single push it on. There is no phone protection case so nothing interfering with power button. Have not used the phone in 2 or 3 years but never any damage...always in protective case and screen protection. No water damage or dropped.