Welcome to Android Central! Did the battery run out even while it was plugged in?
Plug it into a wall outlet and wait at least 4-6 hours before even trying to power it on. A really depleted battery needs time to get enough charge to be able to even turn the phone on. It'd be best to let it charge for 10-12 hours undisturbed, then try again. If it still doesn't turn on, then the next step is to try a new battery, since the current one might have failed. You could also try a different charging cable/wall plug, since those components can also fail.