All great advice ,except for Kathy's! Don't run, but do repot , and loosely pack the moss, and use styro peanuts ,both for bottom drainage and add a few to the mix....to create air pockets.

You can't just water once a week...you have to water when the pot needs it. If it is in plastic ,you should be able to tell the pot is much lighter in weight. Also, if it is in plastic, you can burn or drill some holes into the pot,again for extra air(orchids love air). Also, if the orchid has lost a lot of roots...don't put it into a pot too big...it will stay too wet. Good luck!