Hello Nutmeg, Welcome to Binky Bunny!
How long has it been that your bunny has been hanging out in the cage? Sometimes bunnies like to just "chill out" in their homes and be comfy and sleep. Also, bunnies are "crepuscular" which means that they are most active at dawn and in the evenings – is her cage open all the time? Or only at certain times? If its only at certain times – try making sure its open in the early evening when she’s most likely to be active.
It could also be something medical related – is your bunny eating and pooping normally? This is one of the primary things to look out for when checking to make sure your bunny is healthy – that your bunny is eating, and that the poops are looking nice and normal.
Also, sometimes bunnies take awhile to get used to change, has there been any changes in your house recently? She could be a little shy from that, and to help her out you can try sitting by her cage, just petting her, talking to her, or reading a book or watching TV so that she starts to feel more comfortable! Also, hand feeding her some small treats from time to time will help her feel more comfortable – then maybe she’ll hop out of the cage to hunt you down for some more treats!
How long have you had Nibbles for? Have you noticed any other changes in her behavior other than staying in the cage?
Again, welcome!