I'm playing LeafGreen, and when I defeat Elite4 after credits and stuff, my game starts over, and I don't know how to continue playing on my old "profile". What do I do? :/
You go back to Pallet Town but otherwise you have the same Pokemon and items and everything else. Go and talk to Oak and I believe you can venture outside of Kanto
What Hicky said, but otherwise you could be playing a glitch rom-hack which have been known to crash after the credits.
The first thing one must do before you leave the region is catch I think 50 pokemon to get the national pokedex, then the new quest line opens.