We use "한때" as a noun meaning "one-time". For example, She was once beautiful. = 그녀는 한때(는) 아름다웠다. He was once a member of the National Assembly. =그는 한때 국회의원이었다. "했을때" is used for explaining a situation when something was happened before. 집에 도착했을때 = When I arrived home,~ 내가 한국에 있었을 때 = When I was in Korea,

Sometimes, they can mean the same thing in some contexts. Use the site linked on below for finding the usage of Korean words. http://endic.naver.com/search.nhn?&searchOption=all&query=%ED%95%9C%EB%95%8C