Hi @louis68
Does the phone still charge OK
If it does ensure that the phone is '''fully charged''' and then perform a [https://support.t-mobile.com/docs/DOC-25899|hard reset] (or master reset as it is sometimes called). Scroll down to the section headed ''Master reset with hardware keys'' and follow that section ''only''
''Be aware the a hard reset will erase all your data and downloaded apps. It will restore the phone to its' factory default condition. It will be like when you first bought it.''