The plant is declining because it is, and has been, without healthy roots, and has been unable to take up any water.
New roots only come from new growth. The plant can live for a while without roots, living off water and food stored in the canes, which is why the canes shrivel.
Pot it in the smallest pot possible, water when the potting mix (orchid bark mix) has just become dry.
It may, or may not survive. If not, don't be hard on yourself - most of us have killed our share of orchids.