Flip her over and dribble a bit of Maple Syrup (the real stuff) into her mouth. The high-sugar and sweetness should trigger a hunger response.
If she takes it, offer some apple/banana/soft food, water (via syringe), and more syrup if necessary. Don't drown her in it, just a tiny amount. Make sure she's kept warm (but NOT HOT), and feels safe and secure.
I had a four month old doe completely COLLAPSE on me. She dropped weight, she barely ate, she just didn't look so great. One day she just went down, when I pulled her out I thought she was dead at first. When I brought her in, she couldn't even lift her head. After ten minuets she could raise her head. After twenty minuets she was sitting up on her own. After half an hour I had to move her because she kept trying to climb out of the box I had her in.
She's now bouncing all over the place and relaxing with her brother using the method above
Gaining weight impeccably again, has her appetite back like she should. She's always been happy and healthy, don't know what caused her to drop so drastically
What kind of hay and pellets are you feeding? Is there any corn in the pellets? To much corn in a rabbits diet can cause health issues.