You haven't given much to work on. The two big killers of rhododendrons are wet and dry. Too wet causes root rot. Too dry causes die-back. Other possibilities are weed killer on the lawn, lime on the lawn, the loss of a shade tree, or being improperly planted 20-years ago.

That is probably the most likely. If the roots weren't opened up before it was planted they may be strangling themselves now. You would have to dig it up to detect this. If that was it, you could cut away the offending circling roots. It would be a real challenge to the plant, but it has a possibility of recovering if it isn't too far gone.

Did it just go above the level of your roof and lose shade? Where I live you can see the clear difference from after a rhotodendron passes up through its nice hot shade into the sweltering ultra-transpiring heat in the sun.