Sounds like a good case for your vet. If this hfr is down and can not stand then there is something more wrong than what milk replacer can fix.
How old is this hfr?
Where is she located? Out on grass, dirt lot, concrete lot.
What had she been eating? Fresh green grass, hay, corn, distillers grains, wheat pasture.
When was she wormed last?
Does she have a fever?
If I was treating this hfr, I would lean heavy on the therapeutics. If she doesn't have a fever then I wouldn't give her an antibiotic. But I would throw a barrage of other stuff at her. Multi-min90, Vit AD&E, Vit B, Thiamine, Bannamine, IV glucose/dextrose.
If you have access to a stomach tube and pump, you could give her bluelite, propylene glycol, and warm water.
Please give us some more details. Good Luck.