The apps can't found Apple TV. Already checked both are on the same network. And I use it before. Don't know what's going wrong.

Answers can only be as helpful as the details provided, and there is very little hear to go on. What happens when you try to connect? Is there an error? If so what does it say. How are the devices connected? WiFi, Ethernet? Which ver of ATV do you have? Best guess is that you are using WiFi and the probably go away if you use a wired connection to ATV.