Phone is dead after not using it for a long time.

Do you have a Voltmeter to check if the battery is holding a charge? if so disconnect the battery then test the positive and negative post. The voltage should be printed on the battery.

I would say though that you let your battery stay dead for too long and needs to be replaced, just as if it was a car, or powersport battery. All batteries if they sit to long can not be revived.