There are many reasons that contribute to plant die back. Sometimes the plant is severely attacked by insect pests, bacteria or fungi. Sometimes plants are injured by trampling, over crowding, over watering, under watering, over grazing or even salts or chemical spills. The remedies for such causes vary and might take some time to see results, but perhaps more often die back results are not as drastic and the solutions are quite simple.
For example, a particularly harsh winter, a misplaced drip line or a neglected watering schedule might cause an otherwise healthy and happy perennial to partially die back. It's unfortunate, but it does happen, especially in a harsh environment such as the high desert where growing conditions are not always what we might expect or hope for. In such cases, it is probably better to think of the plant's condition as a mere set back rather than a more drastic die back.
In a case like the one pictured here, the set back results are such that the plant's life isn't actually threatened. In fact, the plant is very healthy and ready for and responding to the coming growing season. But as it sits (in the 'Before' photo), the plant obviously needs a little beautification, an easy task. All that is needed is a little brushing away of the dead debris and voila! The signs of its set back are no longer visible, the plant is beautiful once again and visitors will not even know the set back occurred.
Sometimes eliminating the signs of die back take clippers or other pruning tools, but it's a task that has its rewards in a garden that looks healthier, happier and well tended.
It's also important to find the reason for your plant's distress since not all causes will just go away on their own. If you find signs of disease or insect attack, definitely visit the nursery and talk to the experts.