2 year old havanese won't go outside?

my dog lola has been trained to go outside with our other dog rocky (5 year old wheaten terrier). but the past few days she refuses to go outside, and if she goes down the stairs she just won't touch the grass. usually when it's really cold on rainy she does this but it isn't raining or really cold. we finally got her to pee once last night but since then nothing. we've walked her, stood with her outside..tried everything we can think of. any suggestions?