It sounds to me like your player's file system is corrupted. You might first try to format the players flash and see if you can then load and play back files. Before you format your player, make a note of the folder/directory structure, you will probably have to recreate it later after formatting.
When you disconnected your player - did you click on the "safe to remove hardware" icon in the lower right hand corner of windows? (I'm assuming that you are using windows, if you are using a mac it works differently). If you did not click on this icon, you really should and follow the instructions/stop the appropriate drive volume. The reason you should use this windows feature is because windows may indiciate that it wrote all of your files to your external device but in reality it is slowly writing them out in a background service. Clicking on this icon and selecting to remove/stop an external device will cause windows to finish writing out files to the external device and properly and properly release the file system. If you did do the "safe to remove hardware" icon, please ignore this public service announcement
Ok, if you tried formatting out your drive and you still have problems then it is possible that you have a problem with the firmware or perhaps some kind of hardware failure.
I hope some of this helps,
David