Level Resets

Guys when i test my game on android phone like samsung and press back button then levels are resets automatically and starts from 1st level how can i fix this problem

And also this is big issue for me becaue gameover ad is full screen and when players press back button to get rid of ads then levels resets

 

Any body??

 

Still problem
Somebody

 

If the player finishes the level, the game is saved?

 

There is no world select
Game is like phases with checkpoints and my problem is i have full screen ad in game over screen and most of the people use back button to close ad then it implement not only ad it also cover game over then it starts from start scene

 

The function of the Restart button or Play button is, "Restart" or "Restart checkpoint"?

 

Restart button is restart check point and main menu start button is start checkpoint