If you chose to power down your SP, then all you need to do is to go to the wallet.Click on the arrow which is at the right side of your Steem Power amount,then choose the option 'Cancel Power Down'.

The power down will stop.

For further information read this article:

https://imwatsi.github.io/utopian.io-help_center/tutorials/powering-down-your-steem-account.html