Why my Nikon doesn't auto focus anymore?

I have a problem with my nikon D5200, it doesn't auto focus anymore, with a day ago it was just fine and worked perfectly and now it doesn't even show those little squares when you look into the ocular, it doesn't make them red as the focus is bad or something. It just died.
The manual focus works good, I have to mention its new, I haven't used it much… Just 2-3 times, and yes I have set both lenses and camera body on autofocus mode…
A few days ago I saw some dirt on it (inside the camera body) and I've cleaned it gently, I don't know if that could cause such a thing…

Are you using the lens that came with the camera? The D5200's body does not contain an autofocus motor. That's fine if you intend using Nikon's later AF-S lenses, which have their own autofocus motors, but not if you have older AF lenses, where the focus mechanism is driven by the camera body.

Do you have a charged battery?

If you answer "yes" to all the above, unless you unintentionally have it set to manual focus, you would need to have the camera service.

Check that the button on the camera AND the one on the lens are both set to auto focus.

Dirt shouldn't be inside the camera body… That suggests bad practice or technique when you are changing lenses.

The only sensible advice I can suggest is to give both the camera lens contacts a good wipe and do the same with the contacts on the camera… Dirt can stop the camera and lens 'communicating' with each other… There really shouldn't be dirt around these parts… I'd also pull the instruction manual and check the 'settings'… You might have the camera on a setting that doesn't 'autofocus'… I can't help you much more than that 'autofocus' doesn't interest me (sorry)… I only shoot in full manual, both focus and metering.

If your problem remains… Then you need to approach your camera dealer and ask for advice.

Why doesn't the autofocus work on the 70-210mm D I just bought?