First thing, check her water. If she has a bottle, make sure it is working well. I suggest giving her a crock of water as well, just for now. If she doesn't drink, she won't eat and will go downhill fast.
Second thing, check her poops. Are they normal? Any sign of diarrhea, bloated gut? Does she seem to be in pain... sitting hunched in a corner, grinding her teeth? She could have a blockage.
Check her teeth. Bad teeth can keep a rabbit from eating.
Offer her some grass hay, some kitchen oatmeal (like Quaker Old Fashioned, not instant). Offer small quantities of fresh greens - dandelion leaves, parsley, romaine lettuce... just to try to interest her in food. If you suspect a blockage, a chunk of fresh pineapple may help.
It is vital to get her drinking. Try offering her water with a bit of apple juice in it, or chamomile tea. Make sure she has plain water as well. She may need an electrolyte solution, such as children's Pedialyte.
Observe her carefully and report back so we can give you advice tailored to the situation. I'm only mentioning some common reasons for a rabbit to quit eating. Oh, perhaps most important... Look for what has changed. New bag of feed, different brand, different bale of hay... that kind of thing. Sniff the feed and examine it for signs of dampness or mould. These things can not only put a rabbit off its feed, they can also make it sick.