default language problem

hi
i have a site in english and french. first my default language was english and working fine. For english, i did not get any language prefix in url and for french the prefix was fr. I did not face any issue in this.

But now I need french as default language. I change it from admin panel language settings. But now default french is working fine. and english is not working. I dont find any language prefix for english as well as for french.If i click a menu item it opens article in french correctly.but if i click english language now, it product "Page not found" error.

Please help me .Thanks