Pretty sure I read this as a paperback in the early 1960s. Benign humanoid aliens searching for advanced civilizations crash in prehistoric Europe(?). The ship's anthropologist on his death bed tells them the best chance of ever getting home is to travel to the North American continent and hibernate for many thousands of years, hoping Earth will have developed space faring tech by that time and most likely on that continent. I think they wind up in a cave in Colorado where a hiker (I think an advertising guy whose life is falling apart) is captured by them as they wake up in modern times. Eventually they figure their best bet is to go back to sleep for another 500 years and they are joined in this by their captive, now friend.