The battery is dead. It was working fine before the battery died. It's been used about 5 times. You can replace the battery yourself. The battery type is CR2450. However, just buying and replacing the battery can lead to water damage during use, so it's recommended to use a replacement kit that includes a new back cover, O-ring, and silicone grease. These kits are sold for around 3000 yen on the Aqualung official website, diving goods shops, and Rakuten, etc.