Certified GKH Gardening Expert
It is possible that they have died. This can be common during wet, or unseasonably cold winter, like has been in my area the passed two years. It helps to wait to cut them back until spring, to give them a little extra protection from cold.
If you suspect that they are dead, or the center of the plant is mushy or decaying, then it will be likely that you need to replace them.
First, I would add dolomitic lime and wettable sulfur to the area to kill off any infection, and reset the soil to a proper pH. Then after two weeks or so, you can replant.
This article will help: https://www.gardeningknowhow.com/ornamental/foliage/pampas-grass/growing-pampas-grass.htm