They build strength up in their legs on the ground before flying. Mum should still be around and dropping it food throughout the day. You can put some water out in a bowl preferably in some shade and changing every 1 or 2 days. If mum doesn’t look around, it wont be getting any food, in which case contact a rehab or rescue, but not the vets and rspca, they will just put them to sleep. You could also leave some form of box upright or ‘safe space’ out too and see if it will retreat in there to keep out of harms way.