Why is my IPhone 4 is the only apple divice in my house that won't connect to Wi-Fi'?

Why is my IPhone 4 is the only apple divice in my house that won't connect to Wi-Fi'?

Added (1). Why is my IPhone 4 is the only apple divice in my house that won't connect to Wi-Fi'?

Reset network setting

Go to Settings, and ensure that, Airplane Mode is off. Then open General, Reset and select Reset Network Settings