The watch doesn't turn on no matter what I did

The watch all in a sudden started blinking, then an X. Battery level is around 40% at that time.

Then charged it and tried a few times to turn it on. Been charging it for a long time too. Still doesn't turn on.

Tried to long press the button, no response.

Pretty sure it is charged (unless that's the part broken), don't know what to do to turn it on anymore...

Thank you no matter what!