If you remove the cover and you have a soldering iron, you could desolder the battery cables, charge it with a source or if not, a charger without a tab, connect the positive of the charger with the positive of the battery and the same with the negatives. ... you expect it to charge for a while, you measure with a multimeter that the battery has 3.7 volts, you re-solder the battery to the board and there it could turn on ... if it doesn't charge, it's the battery, if it charges and doesn't turn on , it could be the power button or the power management ic