In case, if you have kept backup then you can restore macOS from that backup. There are some other ways also to re-install macOS Sierra, one of which is through Time Machine backup. Select Reinstall macOS, click on Continue button, then follow the on screen instructions.After the disk is erased, choose Disk utility then Quit Disk utility.Always keep back up of data in any storage device.
Note: Erasing the disk removes all the info from the disk.