I cannot stress enough how important it is to keep the rabbit's water bottle full, or at least to make sure that the rabbit always has access to it.
One day, a little while ago, Gracie still had food in her bowl from the day before, which isn't like her at all. Her water bottle was full. I put my finger to the nozzle and coaxed water out of the bottle. None came, so something was wrong.
I took the bottle in the house and unscrewed the nozzle. This bottle (the glass one) had two balls inside, so it doesn't drip. Well, the second ball got stuck so water couldn't flow.
After I got the bottle fixed and put it back in her cage, she drank for literally a full ten minutes or more. After she was done drinking, she ate and ate and ate.
Rabbits generally drink 1/2 cup of water per day. More if there is a lot of salt in the diet or in the summertime. My rabbits probably drink over half a cup because they get half a cup of food per day. Naturally, they should drink more then they eat.