It would be far easier for you to have stated the make, model and exact symptoms of the fault in your original question. It will also help that you title the question with a few keywords e.g. make and model. You have asked the question in the fix MP3 category and so we know you need a fix and we know you are asking about an MP3 player but your title adds nothing extra. You may not be a lazy person but your title screams “I am a very lazy person and will not help you to help me”.
The fix will depend on the make, model and exact symptoms of the fault. Also if you can report on the software/firmware version that often also helps others to give constructive answers.
Giving accurate and pertinent details may help someone to identify your issue and give an answer you will be happy with.
If someone (in this instance you) cannot be bothered to give such details why should someone list the many thousands of possible fixes for all make, models, software versions and the various ways that files cannot be accessed? Even if someone did list many of the thousands of possible fixes would you really be willing to try them all out?
If you are serious about asking for help to fix your MP3 player then re-ask the question but add the make and model to the title, and within the question describe what happens when you try to access the files, and also what you have tried so far. Also consider what else you could state that could help someone to identify the cause and suggest a fix.