When your Nest Protect ($105 at eBay) is checking in with its app, you can use the app as a remote control to silence alarms, check on your home when you're away, and get monthly summaries of its use.
When it doesn't connect with the app, those things don't work. The worst part is you'll need to climb up on a chair to stop the alarm instead of just tapping a button on your phone when you burn something in the kitchen.
Here's how to get your Nest Protect connected again so everything works the way it should.
Check your app
The first thing you should do is check if the app on your phone is outdated. Go to either the Apple App Store or Google Play to be sure that you don't need to update to a newer version of the app. A quick update may be all it takes to fix it.
Test your connection
Another reason why your Protect may not be working with the app is because it is no longer connecting with your router. There are a few things you can check.
First, try resetting your router. Unplug it for 30 seconds, then plug it back in. When your Wi-Fi comes back online, go to the Nest app and tap Checkup. Be sure you're standing beside your Nest Protect and tap Continue.
If it's still not connected, there may be something in the way interfering with your connection. You may need to move it closer to the router so there's no interference with the signal.
Nest protects also can't connect to 5GHz networks. So if you have dual-band router, try switching to the 2.4GHz band. To do this, go to Settings, tap on your device and tap Remove Nest Protect. Then go to Settings > Add product and then follow the onscreen instructions. When you get to Wi-Fi settings, make sure you connect the unit to the other network band.
If nothing else works, your problem may be your router settings. Nest Protect isn't able to connect to a router with certain parental controls or to routers with firewalls, for example. Here's a list of Nest's recommended router settings that can help.