Sometimes the rootball becomes so dry that it is very difficult to rewet it. Remove the plant from the pot and check. If the rootball is very dry, soak in water for 30 minutes or so, then replant. Misting the leaves can also help. The other, more ominous possibility is that too much water and/or less than ideal drainage in the pot has caused the roots to rot away. Unfortunately, there is very little that can be done if this is the case. A third possibility is that the roots were killed by low winter temperatures. Rhododendrons which would be perfectly hardy planted in the ground can be badly damaged or killed by the low temperatures experienced in an unprotected pot. There is no cure for this either.

The Rhodo wasn't in a pot over the winter - it was in the ground. I potted it up yesterday when nothing seemed to revive it where it was. I didn't notice the root ball dry when I potted it up but it could be that they rotted away. I will have to check that possibility. Thanks for your comments.