Recruit Shadowheart on the Nautiloid or on the beach
Keep her in your party and avoid dismissing her
If she is in camp, speak to her and choose the option to add her back to the party
If she is not in camp, check your party management screen and swap her in
If she has left due to approval or story choices, reload an earlier save if needed
If she is unavailable because of a quest state, progress her personal quest until she rejoins
If she has been knocked out or downed, use healing or revival to restore her
If she is dead, use a Revivify Scroll, Withers, or a resurrection service
If she was lost due to a bug, reload a prior save or restart the game
If you mean her from a locked area, complete the area objective and exit normally