I have my old account on level 18 however by mistakes got pressed the for new game now how do I get..

If you linked it to a Google, Facebook or IMO account. you can simply log in and your progress will be there. If you didn't, there is absolutely nothing you can do to get it back i'm afraid.

As mentioned once you get access to the setting menu you can log in again with your previous detain, IF you had the game bound to an external account.

If not then the game developers do have an in game support contact form where they say you can post details of your previous account and they may be able to help you recover it. But I don't know how often this works. Check How to get help in the game for more details.