Actuallly the matter is that i have downloaded an mp3 file from internet and it automaticaly got converted into MS-Dos application file .now how can i convert back it into mp3 file

Are you saying it is opening with DOS or file type is changed.
If file is just opened with DOS but extension of file is mp3.Then just right click and select openwith and select any of media player Or you can change default opening of files with similar type.Right click,select properties.Change Opens with by clicking on "Change button".

If file type is changed then:-
Go to folder options->View.There uncheck "Hide extensions for unknown file types".Now you can see the file with extensions.Just rename the file and after last dot remove that part and add mp3.
Check if the file you downloaded is valid mp3 file or not?

It is valid Mp3 file and its extension is also .mp3 but the problem is that it does open in any media player

First, make sure you have view extensions enable as IIM said above.

Does any MP3 open in media player with a double click? IF not, then your file association has changed. Easiest way to fix is to use Windows File explorer (not internet explorer), right click on the mp3, choose 'open with', select 'Choose default program'. Now find and pick the media player for your MP3 files.