Rhododendrons have dormant buds under the bark. If the branch is still alive, it can recover. If a branch is alive, the area between the bark and the wood is green and it will start sprouting new buds with leaves. When the branch dies, the green layer disappears. It is good to cut off areas that are known to be dead. If you aren't sure, it is wise to wait a couple months.

This dieback is usually caused by drought, or borers. In the east this year we had a very dry but mild winter followed by a hard cold snap. Many very hardy rhododendrons had a lot of dieback after this. It is unusual, but this spring we saw a lot of it.