Sound no longer working on my monitor?

I have both my 32" TV and my monitor connected to my PC at the same time, but normally I only display the desktop on my monitor.

Yesterday I wanted to watch a DVD so I set the option so extend the displays and to make the TV my main display, i also went into the sound options and set the HDMI device so the sound would be on the TV.

This morning however when I try to change the sound back to the monitor I get a message telling me that the peripheral isn't plugged in, the thing is though it is plugged in and I now cant get the sound to go through my monitor again.

How doI solve this issue as it isn't showing up in the sound options dialogue box.

UPDATE:

What I have done is remove the audio jack from the back of the PC and plug it into the headphone socket at the front, so I can get sound on my monitor but for some reason it will not detect anything when it is plugged into the back of the PC even though it worked fien yesterday.