虽然这个地方填what是没错,但是,这个句子本身就有些问题,前提是你没有打错标点.我们可以把它改写为下面两个同义句：
What is left to them now,the people ,who one had fame and fortune,is utter poverty.
These people,once having fame and fortune,what is left to whom is utter poverty.
另外一种可能就是你忘了一个句号或者连词.
These people once had fame and fortune but now what is left to them is utter poverty.
These people once had fame and fortune.Now what is left to them is utter poverty.