Lighetto, currently can it be detected by the MP3 Utilities Upgrade Tool?
If yes, then you will need to try all the firmwares for your player's screen size in the downloads section.
i'll suggest that you open the player and look for the 2nd largest chip and identify the serial number. Example ATJ2085, this way it would be easier to locate a likely firmware for your player.
If the player is not detected by the MP3 Utilities Upgrade Tool, then you will need to perform the Dead Player Recovery
Here.