It might be a problem with the MTABLE.SYS file on the player that stores the song database. If you delete it, the player will reconstruct it. It might be a problem with the tags. Are your files mp3 files? If so, are the tags in ID#V2.3 ISO8859-1 which the player needs? If it is neither of those, and you have copies of all the music in the player on your pc, then the next thing to try would be formatting the player using the player’s menu, which deletes all the music on it. Then you could reconnect the player and copy the music to it again.
As for the player not connecting, after a firmware update the USB mode is reset to the default. You should manually reset it to what you were using before. I assume it was set to MSC if you don’t use protected files, or to MTP if you do use protected files. If you set the USB mode differently than before, then the music you transfered before using the other USB mode won’t show up when you connect the player to the pc. That is why it is important to choose either MSC or MTP mode and stick with it. The default is autodetect, which might connect with either. unless one needs to use protected files, they should have the USB mode set to MSC, which treats the player as an external drive(and the card in the player as a separate external drive). MTP mode treats the player as an MTP device. MTP device mode is much more likely to cause problems than MSC mode.
If you must use MTP mode due to using protected files, then click on the link below.