Ours was very healthy looking for several years but has started to die out with only a few healthy shoots and plumes remaining.
Why Would Pampas Grass Die Out
Certified GKH Gardening Expert
There can be a few reasons for your plant loss. Drought can greatly effect Pampas Grass, and it does need water during dry times.
The plant sometimes will die off from the center if it is to tightly growing together.
Also I should mention that we had numerous reports this spring and summer of Pampas Grass struggling after the long harsh winter the nearly the entire US suffered. This certainly may be some of the effects you are also seeing.
Here is a link to refresh you on the care requirements. Pruning back may help rejuvenate you plants.