Nikon D5100 isn't focusing when using the flip screen?

Nikon D5100 isn't focusing when using the flip screen?

Added (1). I've had my Nikon D5100 for a few years now and kept it in great condition. Just recently I've been having trouble focusing with it. I did a complete reset and put it back to factory settings, and I'm still having a hard time. Whenever I use my flip screen and change my viewing from the view finder to the live view switch, it just won't focus. I keep getting the red box, and it won't let me take pictures. I have no idea how to fix this. Any idea what I can do?

Using a digital SLR as a glorified P&S (using the LCD) is probably not a good idea in any case.

Why? When you hold the camera next to your face, with both hands holding the camera, there's very little chance of camera movement causing blurry photos. When you hold any camera at arms length, especially one that weighs over two pounds, you introduce a whole lot of camera movement and thus blur.

There could be a problem with your setup.

Is the camera set to auto-focus continually or just for single shots? When waving the camera around at arms length, you need to be sure that the camera is in the continuous auto-focus mode. You can assure continuous focus by pressing and holding the AF button behind the shutter release and to the left of the command wheel the whole time you are preparing to take your shots.

Too near, too dark, AF switch is in manual, or camera is off?

As the fault only exists in live. Iew the most likely fault is with one of the AF systems. Your camera has two of them which work in different ways, contrast detection and phase detection. Contrast detection is performed solely using the image sensor and phase detection uses the SLR mirror to split the incoming light path. In live view the mirror is flipped up out of use, so your fault lies with the contrast detection AF.

I suggest that you delve into the camera menu to see whether you have somehow turned off contrast detection AF. If you haven't then the system itself is faulty and you can either get it repaired or live without it and use manual focus when using live view. Repairs won't be cheap, but in many countries Nikon offers fixed price repair menus so you can at least look into the likely repair cost compared to selling your camera as is and replacing it. The fault may not affect the value greatly as many dSLR users never use live view at all.