Question
I came back from a long trip (4wks) and found one of my knockout roses full of branches with dead leaves.  Only a few branches still had green leaves on them.  There is a healthy knockout rose on either side of the one that is nearly dead.  They have a few yellowing leaves, but no sign of whole branches dead.  I looked for pests and saw no sign of a major infestation.  The rose on one side had a mass of leaves/webs caught up in its branches but it is otherwise healthy looking.

Answer
The near dead rose has gone under stress for some reason, such as hot weather,lack of water r eve too much water and has died back. You can bring it back to a normal rose bush. Cut the rose way back past the dead cans, even down to 3 inches if necessary, as it will grow back. Give it a little fertilizer, plus a good soaking of water and in a couple of weeks it will put out brand new growth and then flower for you. One never knows why one rose fails while others do okay because you can't know if one rose is weaker than another when you buy it. This plant was obviously a weaker one. Keep an eye out for it as it may fall back again if conditions are right.