i formatted it again with 'pctools' software that was available for download from the dnt website. and now it works.
I have 'Mini MP3 Player' from DNT. http://www.dnt.de/index.php?dir=details&pid=53032&cat=mp3&m_id=mp3&h_curr=
Today when I plugged it into my computer, the computer said 'drive not formatted..do you want to format it now?' and I formatted it. Now even if there is a mp3 file in the player, it displays 'No files' and refuses to play it. All the menu system is working alright and I can however still use it as USB stick to copy files, but it can't used to play songs. In "About" section of the menu, it displays version 2.015 but shows memory 0MB. I will appreciate if you can help me fix the problem.