I attack one of the base in the map and lost my shield. How can I get it back?

If you scout or attack an enemy player in the game your shield will automatically cancel. This is part of the game play so that players cannot sit behind a shield and attack without risk of being hit back.

To shield again you need to set a new shield (the old one is now gone). There is a cool down though. When your shield drops you have to wait a few minutes before you can add another. This will be based on your base level. So if you are Base level 20 then you have to wait 20 minutes.