Without warning and without any impact or damage, it just stopped working, no power, nothing. And it's not a battery issue, the battery was almost completely charged.

Try and reset again. 
1. Slide and hold the Power switch up for approximately 20 seconds.
2. Release the Power switch.
3. To turn the player on, slide the Power switch up then release instantly.
Part 2 - Make sure the player is fully charged. 
1. Connect the player to the PC. 
2. Make sure the computer detects the player. 
NOTE: This part is important:, switch the player to MSC mode, then connect it to the PC.  3. Leave the player connected for approximately 2 hours to charge. 
4. Disconnect the player from the PC.  5. Verify if you can turn on the player.  6. Reset the player if necessary one more time.