@arthurhauf, First thing I would do is connect it to a charger via a USB meter. ([link|https://www.amazon.com/MakerHawk-3-7-30V-Voltage-Multimeter-Voltmeter/|Example on Amazon].)
If you connect it and the phone is drawing zero mAh, you know the phone is not “accepting” any power, which could be a few reasons. You’ll have to open the phone up and I’d start with replacing the battery. A bad battery can cause the phone to be totally unresponsive, and it can also be the cause for the phone not “accepting” power in, because the circuitry realises the battery is dead.
Here is the link to the [guide|139693|battery replacement guide].