There are several options.
If you open the file and then DON'T see the wave (or a very small one) in your window, then there is no (or very soft) content in your file.
If you open the file and the wave is okay, then most probably it's a matter of volume. You can try all volume settings: of your computer by clicking the speaker icon in the bottom right corner of your screen, and / or the volume in WavePad (bottom right corner in the window).
If no result, try to check your sound card in your computer: are other mp3 of wav files okay or does the problem occur everytime you try to play a file in WavePad?
Good luck!
Leonardo