When I Won Pokemon leafgreen it suddenly ended the game was over, then how can I visit Cerulean Cave after beating elite four
You need to get upto the point in which Celio allows you trade from Hoenn to Kanto.
See here for more detail
Go left and Surf. The Ace Trainer blocking your way will have moved from the entrance.
You have to go the the Sevii Islands and complete Celio's gem quest. Which is Finding a Ruby and a Sapphire. The Ruby is Located at Mt Ember in the cave where the 2 Rocket Grunts are talking. Bring it back to Celio and He'll tell you to find another gem. The Sapphire is Located in the Dotted Hole on 6 island. You have to use cut on the door to enter. When you Reach the bottom a Rocket Scientist will tell you a password to there Hideout on 5 island and run off with the stone. Go to the hideout and defeat the admin to find the scientist again and he'll give you the gem. Bring it back to Celio and The guy Blocking Mewtwos cave will dissappear.
It says "The end", but the game isn't over. If you just choose "Continue" when you start up the game again, you'll be back in Pallet Town and be able to visit Cerulean Cave and the Sevii Islands.