DIAGNOSIS The printer has detected a critical hardware error or is stuck in a firmware loop. A solid red LED is a general fault indicator.
FIX Disconnect the power adapter from the wall and from the printer. Disconnect any USB cable. Wait 60 seconds. Reconnect only the power adapter to the printer and wall. Press and hold the POWER button for 15 seconds. This performs a cold boot cycle. The light should flash green/red during boot, then settle to solid green if successful.
IF THAT DIDN'T WORK The internal lithium battery used for memory backup is fully depleted or faulty, preventing boot. Leave the printer connected to its official power adapter for a continuous 4-hour charge attempt. Do not use a USB port on a computer for this.
HEADS UP Using a power adapter with incorrect voltage (not 5V DC) or polarity can cause this fault and permanent damage. Use only the provided adapter.