Remove battery if possible. Reinsert. Connect phone to charger & press the power on button.
If the battery is not removable, turn on the phone, press & hold the power & volume down buttons simultaneously until the screen shuts down & the boot-up animation appears.
If the phone still doesn't work -
Ask customer to box up the phone along with everything which came with it (except the SIM) & Uplands will arrange collection & delivery of a new replacement item.