Mine fixed after I opened services on my computer, clicked Windows audio and then either click start the service or restart the service!
Note from Kimberley : Came into work this morning and the sound that was working perfectly fine last night when we left wasn't working this morning. We tried the speakers on another system and they worked fine so we knew it was some kind of internal issue. Problem, once found, was a quick fix. Somehow gremlins during the night switched the speakers to mute.
Start/ Settings / Control panel / Sounds and Devices/Device Volume/Unchecked Mute.