If you've received a push notification saying your system has been offline for 30 minutes, it means that your Sync Module is not connected to Blink's servers. This can happen if you've made a change to your network, or lose power or Internet at your home. If you tap on that notification, you'll get this screen:
If you've recently made changes to your WiFi network, like a new router or you've changed the password, you'll want to tap the Change Wi-Fi button to reconfigure your Sync Module.
You'll need to verify that your WiFi at home is functioning, and that your Sync Module has power. When your Sync Module is online, both the green and the blue LEDs will be lit. If both LEDs are not solid, try power cycling your Sync Module.