Dear gogomanu,
if you have access to an
identical working player, you can try to clone the firmware from that using the utility "S1clone" that you can find togheter with instructions on website
www.s1mp3.de.
It normally works well...even if it quite slow, because it extracts and reflashes a raw image of the player memory; not pay too much attention to the warning messages that may appear during the rewrite process.
Good luck!