Speaker stops working after bluetoothing to stereo?

Hello, I can't seem to find an answer anywhere for this but my iPhone speaker seems to stop working after I connect my phone via bluetooth to my car stereo. I visited an apple store to get this sorted previously but as soon as he plugged it into a compuser it was fine, if I do it nothing changes? as I can not be bothered to travel all the way to the store again how do I fix this?