if i minimize game and come into home screen in unity then game always restarted from start.

I assume you are talking about a mobile game.

As far as I know , you can't really do anything about this. Operating systems on mobile devices are very aggressively managing apps to save processing power and battery life. If the OS decides that an app, game or not, has been in the background for too long or other apps in the foreground needed more RAM, the OS will force the background app to shut down to free resources.

The only thing you can and should do is giving your app proper entry points and quick loading times. This way, a restart will be less annoying to the user. A quick loading screen of just a few seconds, then getting back into the game more or less where it was left.