Start by giving her a thorough exam. Check her crop (hard, squishy?) and abdomen (squishy?). Look for parasites and injuries. Check her vent (clean?). Look inside her mouth for anything unusual and pay attention to any sour smell. Feel her keel bone, and see if she has any flesh on her breast.
Does she usually lay regularly and when was the last time? See if you can gently feel a possible egg stuck inside.
If she is really weak, it would probably be best to isolate her in a crate or large box, somewhere quiet and warm, so you can monitor food and water intake while you try to determine what the problem is.
The more info we have, the more we can help.