The Shield Bash action can knock a target prone - however I can't find any effect of being prone apart from the fact that it takes a manoeuver to stand up again.

My guess is that until you stand up you should suffer from the exposed condition or something - but a precision on the matter would be nice.

If you attack from being prone...use the fortune dice and make a jugement based on the conditions...the npc's or monsters in this case would receive fortune to hit him while he's down.

Likewise, if he can't get up for some reason...misfortune to attack while prone is appropriate.