What can have caused my computer speakers to have stopped working.?

They are making a continual buzz when switched on but no other noises coming from them. I've checked that there are no loose wires and everything seems to be O.K. there so does anyone have any suggestions as to why they are not working? Thanks.
Update: Lawrence, they have not been UNplugged. Everything is still in the same place it was when the system was set up for me 11 months ago.
Update 2: Sara, thank you. I could not find Device Manager but did find Sound & Audio Device which had a checked MUTE button. I unchecked it and everything now works fine. I have no idea at all how that was checked and I guess it will remain a mystery but, at least, I now know how to correct it if it happens again. Thank... show more Sara, thank you. I could not find Device Manager but did find Sound & Audio Device which had a checked MUTE button. I unchecked it and everything now works fine. I have no idea at all how that was checked and I guess it will remain a mystery but, at least, I now know how to correct it if it happens again. Thank you for your help and thanks to all others who answered my query.
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