I agree with everything Lindsay said, but may I also add that Dendrobiums don't like to dry out as much as some other types of orchids, either. They have much finer roots than Phalaenopsis or Cattleya, and mine thrive on being misted every day when the weather is warm.
If you left it outside this past weekend, when it was SO cold, that alone might explain the yellow leaves. But don't panic, it will very likely recover if you keep it indoors when it's cold, and give it extra water when it's outside. These like a fair amount of sunlight in my care - my orchids are all inside my pool cage, though so the screening provides a tiny bit of 'shade' relief from the direct sun.
