Excerpt from https://docs.puri.sm/Librem_5/Troubleshooting/Charging.html
Evergreen
If your phone fails to charge, or discharges while connected to a power source during use, follow these steps:
Unplug the phone from the power supply.
Enable all kill switches to cut power to the modem, WLAN, camera and microphone.
Remove the battery.
Press the Volume Up button and hold it.
Plug the provided USB C charger into the phone. The indicator light should flash red.
If the light remains off, try repeating steps 1-5 but with the USB C cable plugged into the phone in the opposite orientation.
Carefully insert the battery.
The indicator light should be a constant red and should not flash.
Wait 1 second then release the Volume Up button.
Unplug any USB device that might be connected to the phone
Wait at least 1h for the battery to get some charge
Press the power button untill the green indicator light shows and then release the power button