DIAGNOSIS The battery is not providing power. This is a known failure mode. The internal lithium-ion battery can degrade or the charging circuit can fault.
FIX First, confirm the charger is working. Use a USB power meter or test the cable with another device. Connect the P21 to a known-good 5V/1A USB-C charger for two hours. Press and hold the power button for 5 seconds. If still dead, perform a hard reset. Press and hold the power button for 15 seconds while connected to power.
IF THAT DIDN'T WORK The battery has likely failed and requires replacement. The P21 uses a proprietary 3.7V lithium polymer pack. You must open the case to access it. This voids the warranty.
HEADS UP Do not attempt to use a fast charger (e.g., 18W). The P21's charging circuit is only designed for standard 5V input. Using a PD or QC charger can damage it permanently.