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This is a very big deal in a rabbit, as you may have read about online on the good sites that have rabbit information. Regardless of the underlying reason she stops eating, what then happens is the digestive tract comes to a halt (gastrointestinal stasis). This then triggers a series of negative events including overgrowth of the bacteria that lives int he digestive tract. With this overgrowth there is excessive gas formation in the gut and then pain which furthers makes the rabbit uncomfortable and unlikely to eat. Following this the bacteria release toxins (endotoxin) that can cause a rabbit to become quite ill and die.
Okay, that being said, if it is possible to get her in to be seen by a veterinarian that is confident with rabbit care, I recommend this. She really needs fluids and depending on how dehydrated a rabbit is this could be as simple as out-patient subcutaneous fluids or as complicated at in-patient intravenous fluids. This is the cornerstone of getting a rabbit through GI stasis. At home you want to start syringe feeding her. If you have access to the Oxbow critical care recovery diet then use this. If you don't you can use people baby food (vegetable or fruit varieties...I find they like the sweet stuff when they're sick) and feed approximately 10-20ml every 6 hours until she is eating very well on her own (may take several days). You can certainly mix the baby food with the Recovery Diet or a blenderized pellets. Continue to offer all her favorite foods and keep the pellets/hay/water fresh.
Additional treatments that I usually prescribe for rabbits does depend on what I find on a physical exam which is why I recommend get her in to be seen. Some rabbits need antibiotics, pain relief, anti-nausea, anti-gas medication. If suspicion of a hairball comes into play, there is specific treatment for this (this sometimes can be palpated in the stomach).
For a website with excellent rabbit info and articles, if you haven't already discovered it try: www.rabbit.org.
Best of luck.
It is definitely possibly that there is an underlying age-related problem (e.g., organ failure, cancer, etc) that is causing her to stop eating. However, age isn't a disease and just because she is an older rabbit, doesn't mean we shouldn't treat her. She's actually not that old in the general scheme of things. It could just be one of these seasonal things that some rabbits experience when they go through a bout when they don't eat and with a bit of supportive care perk right up. That being said, screening lab work is always indicated in an older rabbit to be sure there isn't an underlying problem. I also do want to be sure she is spayed. If not then uterine cancer is a very likely issue that could cause what you are seeing; there is no cure for this if it has spread extensively. Your veterinarian should be able to palpate her uterus and tell.