While dendrobium orchids have the benefit of water storage devices in their body, they still can easily become dehydrated when neglected, placed in a very dry setting or exposed to high heat or direct sun. As epiphytic orchids, dendrobiums acquire moisture and nutrients from the air surrounding their roots. Potted dendrobiums are planted in a medium that is light, free draining and does not hold on to much water at all.
Drench your dendrobium in its pot once a day for a few days. Run the pot under a gently flowing tap of tepid water soaking the planting medium and lower quarter of the stem. Allow all of the water to drain through to the sink below before placing the orchid back in its decorative pot and/or growing location.
Place the orchid in a location with bright indirect light but no direct sun and no drafts but some fresh air circulation. This will reduce the stress on the plant, reduce water consumption by the plant and allow the roots and stem to rehydrate.
Set the dendrobium atop a tray or shallow dish lineds with pebbles and filled with water. This will serve to raise the humidity of the ambient air to a level more comfortable for an epiphytic orchid. Refill the water as it evaporates into the air.
Repot your dendrobium into a fresh orchid medium once you have introduced some moisture back into the plant. Orchid medium, especially old potting medium, can dry out past the point that it is capable of being rehydrated no matter how much water you apply. Replanting the orchid into a slightly larger pot with fresh medium for epiphytic orchids will help restore the orchid and provide for a more moisture-retentive environment for the drought-stressed roots. Be sure to remove all of the old medium stuck in between and to the roots and run some tepid water over the roots before replanting to make them pliable and easy to spread out in the new pot.