Is it okay for a rabbit to be lying down in her cage?

This depends on a few things. Such as..

Is her appetite and droppings normal?

Is she drinking and does she have fresh water?

Is her cage in a draught or strong sunlight?

Is she breathing normally and not panting?

These are a few basic things to check on for your rabbit. If she seems distressed and not eating or drinking normally you may need to see a veterinarian. Try offering her some dandelion leaves if you can find some or fresh carrot.