Volume won't go up or down

I had an issue with the radio yesterday where it wouldn't locate all the stations I had previosly tuned in. I eventually managed to get them all back, but now the volume is stuck on one level and can't be turned up or down either by the control on the main radio, or the joy-stick thing I have on the steering wheel. I'm assuming I pressed something while tuning the radio again but can't work out what...please help!!!