If your iPhone battery has been in a discharged state since January there is the possibility that the battery may no longer hold a charge.
Leave it plugged in to the charger for several days and see if that makes a difference.
Can you turn the iPhone on when it is plugged in? If so, go to Settings>Battery>Battery Health & Charging and see what the battery health level is.
If that doesn’t work and you live close to an Apple Store , make a Genius Bar appointment to have a diagnostic performed. This should tell if your battery is still capable of holding a charge.