I have had the same problem a few times before as well. There are several possible things:
1)You have installed a windows update and its messed up media player. In that case, just download the latest media plyer and reinstall it.
2)Your sound card is playing up (most likely, as the fact that you cant change the volume means that the computer has detected no sound card). Therefore, go into device manager and check that everything is OK. If there's an exclamation mark by your sound device you will need to reinstall it.
3) I don't know, but I'm sure there are other things...