This is normally caused by one of two things.
1.) Either your Box or TV has been muted
Check this with your remote control handset,
2.) Or, one or more of your Scart leads has become lose from the socket.
Check this with your remote control handset
Check all connections both on your digital box & on your TV.
Lost sound is unlikely to be a reception problem as this would normally affect the picture in some way too.
If you still cannot get any sound we advise you contact your manufacturer's helpline.