If you have Shield Security installed on your site but site is not visible in the Shield Central control panel, that means that there is communication issue with it.
To resolve this problem, you can use the button "List Status and Errors" to see if any of your sites have problems.
If the site is showing as having a problem, you can have Shield Central reconnect to your site to check that connectivity can be restored. There's a button on the Status screen to do this.
This would normally happen automatically in the background - it just depends on the timing after a site outage when you review the status it may need done manually.