D5100 issue, is it me or the camera?

Ok so I have a Nikon d5100 that I'm just learning to use. Sometimes when I'm shooting in manual mode it just stops working. Like when I press the shutter button nothing happens, no auto-focus or anything. Usually when it happens I have been shooting away for a while with no real problems. So could it be that the camera has gotten too hot? Or maybe it just doesn't like the settings that I've chosen (although you would think that it would still take the photo and it would be under/overexposed or whatever) Or do I have a actual problem? The camera is still on warranty but I don't really want to take it back until I know for sure its not just something I'm doing wrong

First make sure if it's in 'made in china' mode.

The camera usually will not fire if in single shot AF mode and the subject is not in focus.

Also, it will not fire if the exposure meter reads HI or LO. This, however, would be in non manual mode.

Other than that I can't think why it is doing that. It certainly won't be overheating.
You could maybe try resetting to default. That may work.

It would be most probably you but if you're in doubt, you can always return to the store and ask for a replacement.

Have you set the focusing mode to continuous and then had the camera on the go for an extended time, that could conceivably overheat a D5100, especially if you were using live view! It's the sort of issue that a D7000 would cope with quite quietly.