Scant information to go on really martyn...but we will give it a go!!
There are really only 2 possibilities & the one is far more likely than the other.
The first is that you have somehow damaged the Windows Driver that is needed to recognise the DVD device. I think this is pretty unlikely as you can still read music from it. If you have damaged it then the simple answer is to re-install the version of Windows you are using.
The second is that you have damaged the software program that you are using to play movies. A little more likely I think. If you have not downloaded anything special then you are undoubtedly using Microsoft Media Player. You could try reloading this product from the net, but Microsoft do some checks on your system before allowing you to download the latest version & to be honest, it is a big file & the player is FAR from the best available. You might try downloading VLC player (from here
http://www.videolan.org/vlc/download-windows.html) & then try to use it to play a movie.
If it will not play (& I will be surprised) then please take note of any error messages!!
) but i seem to have taken the DVD player off some how as it was working before , 
