macOS High Sierra no sound on Macbook air and volume has the prohibitory sign! Help!

So I recently updated to the latest update. and a day later I realise no sound on laptop.I checked sound on system preferences and I have no input or output device. The mute bus is ticked and keyed out so there is no way to untick it. Volume doesn't go up or down just has the prohibitory sign. Please I would appreciate any help