Anytime a rabbit uncharacteristically refuses food that they normally would eat, or is subdued in a way they normally aren't, it is a cause for concern. Most often it is some sort of stomach upset, which is most often a great cause of concern in rabbits due to their reaction to it by not eating, and the possibility of bloat also occurring(gas pressure building to dangerous levels). There are also other possible causes of a rabbit having lack of appetite and unusually subdued behavior, as anything that can cause pain and/or discomfort can cause this reaction.
http://www.bio.miami.edu/hare/ileus.html
Usually the first thing I try when I notice a rabbit not eating is baby gas drops, 3 times an hour apart. If by an hour after the last dose they are still acting unwell then it usually means it's time to get them to the vet. With this having happened to your bun yesterday and your bun still acting unwell today and presumably not eating(? or not eating much), I would be inclined to just get her into the vet before they close today since it's already been more than 12 hours since she had this lack of appetite(not eating for 12 hours is considered an emergency), and tomorrow is Sunday and they likely aren't opened. If you want to give one last thing a try before trying the vet, I would maybe try syringing some plain pedialyte, a small amount(less than 2-3cc depending on the size of the bun), while first palpating her stomach just to make sure it's not bloated(if bloated will feel just like a balloon up under the ribs). If it is bloated, immediately get your bun to the vet. If it's not and your bun has just gotten dehydrated after not feeling well, and this dehydration is why she currently is still not feeling well, this should give the needed boost to get her back to eating. I would still schedule a vet appt, give an hour after syringing the pedialyte, then if she still was acting abnormally, get her to the vet.
The vet should check her teeth to make sure it's not a dental problem, and palpate the belly checking for a distended stomach(bloat), GI obstructions, bladder issues, etc. They may even want to do xrays and/or a blood test(though this can increase costs significantly if that is a concern for you), but usually if nothing obvious is found on palpation they will prescribe the meds for a digestive upset and see how your rabbit responds to that. The meds for you to take home should be a gut stimulant like metoclopramide, meloxicam for pain, and a syringe feeding food mix. I would also make sure they give your bun sub q fluids there as that can sometimes give them just the boost they need to get back to feeling better.
If you have a suspicion pellets may have caused this, why do you think that, did you start them just prior to this happening, and what kind are you feeding her? If you had just started the pellets the day before or day of, then yes they could be a cause. I would only feed a plain grass hay like timothy for now, when she starts eating on her own again and until she is back to normal, then gradually reintroduce pellets back into the diet if you so choose(though possibly a different brand this time). If the pellets could have possibly been the cause, you may need to switch pellets depending on what is going on there.
Also keep a very close eye out for diarrhea occurring. This is when there is no normal fecal balls and everything is watery or pudding like. That is an emergency and you would need to get your bun to the vet immediately. If there are normal fecal balls and some mushy ones also, this is usually the cecotropes and that's not usually as life threatening and may be diet related. After a bout of GI stasis, usually there will be some odd shaped and possibly a bit moist fecal poop, maybe some mucous. This occurs from the irritation to the digestive tract and should clear up in a day or two. Though if you have any concerns always consult with your vet.
