If you don't have a backup somewhere, you are out of luck.  There is no repository of the code on the flagcounter site since you are not even required to register to get the code.  If and when you create a new one, remember to create a backup and put the HTML code away safely.

You need the url of your fc page, something like "http://s0x.flagcounter.com/more/xxx".
With that, you can rebuild your code. If you have luck, it still may be on your browser's favorites or recently visited pages.