This is why rabbits need hay constantly available to them. Their gut needs to be moving all the time, to maintain a healthy GI tract. When rabbits don't have access to hay or pellets all the time, their GI can slow, and a problem can arise called GI Stasis.
GI Stasis can occur whenever the intestines come to a complete standstill. This can be caused by not enough roughage in the diet, a hairball, or something else clogging the intestines. Sometimes it's not enough exercise, stress, dehydration, or pain from another underlying illness. or even sometimes it's genetic. I've heard the Dwarf Hotot rabbit is very susceptible to GI Stasis for some reason.
GI Stasis is a serious problem. It's not very common, but it is very dangerous. When the intestines come to a complete stop for over 24 hours with no care, the rabbit can die.
"An intestinal slowdown can cause ingested hair and food to lodge anywhere along the GI tract, creating a potential blockage. Also, because the cecum is not emptying quickly enough, harmful bacteria such as Clostridium species (related to the ones that cause botulism and tetanus) can proliferate, their numbers overwhelming those of the normal, beneficial bacteria and fungi in the cecum. Once this overgrowth occurs, gas emitted by the bacteria can cause extreme pain. Some Clostridium species also produce potentially deadly toxins. It is the liver's job to detoxify these poisons, at a high cost to that all-important organ. Damage to the liver can be a serious--even life-threatening--side effect of GI stasis." ~http://www.bio.miami.edu/hare/ileus.html
Symptoms of GI Stasis include little or no poop pellets, poop covered in mucus, loss of appetite, lethargy, and pain (being hunched in a ball, loudly grinding teeth).
GI Stasis can be cured. Different things work for different rabbits, but I suggest bringing your rabbit to a vet immediately after detecting any sign that your rabbit may have GI Stasis.
Visit the site I referenced above, for lots more info on GI Stasis.