Continuing on from the last post, I suspect it's got something to do with voltage as well... though you've specified it's been a while since you've charged it, haha! No worries, I do the same with my Smart sometimes. I just can't help playing it while it's charging as well!
A few quick questions who's answers may make the following info pointless: have you ever charged your Smart before this? And, did you plug it into a wall outlet/AC adapter/USB port that wasn't connected to a laptop or computer?
If you've plugged it into anything else other than a computer to charge it: If you're outside Japan, you'll have to check if your country's electrical voltage (for plugging things in through wall outlets, etc) runs the same as theirs (100V is Japan's standard, and the voltage Bandai requests the Tamagotchi Smart is charged with). Like what
@KuroiNeko-Chan said, the Smart and it's cousin, the Digimon Vital Bracelet (both made off the same tech under the same company), are vulnerable to getting fried by international voltage rates if you haven't done your research.
The last page of the Tamagotchi Smart's manual, translated into English by @OldSchoolVPQ and hosted on Gotchi-Garden specifies it better - and recommends AC adapters (not different third-party cables) that, unfortunately, are compatible only with Japanese wall outlets and voltage rates. They did not recommend any other cord other than the cord included with the device, which may be a factor in figuring out what happened to yours!
If you want to charge your Smart through a wall outlet and your country has a different (usually higher in these cases) voltage rate from Japan, you'll have to use a transformer - step-down or step-up depending on whether your voltage is higher or lower than it should be. They're different from those wall outlets that have a different-shaped plug for international travel and/or otherwise - they actually convert the voltage! And are unfortunately quite expensive.
Hopefully this isn't what's happened to your device, but if you do have a history of charging your Smart anywhere other than a computer/laptop USB port (the only thing international Smart users can really use to charge their device without shelling out heaps for a transformer), it may be likely that your Smart's been fried in the process - you'll have to replace it if that's the case. Without more info, I'm not sure what else could be causing your device to malfunction.
