WHy won't my Nikon D90 take pictures?

It won't focus and it shows an F and two backwards 3s. It looks like this: F EE and it flashes.

I've tried taking the lens off and putting it back on. It makes a solid click when it connects. I've also tried messing with all of the different settings. Nothing is working.

This error indicates the lens is incorrectly attached to the camera

Are you using a lens with aperture ring? If so, then set the ring to the smallest possible aperture (largest number), e.g. F/22. That must be done so that the aperture lever at the mount of the lens is free to move all the way when the aperture is controlled from inside the camera.

Two options

1. The lens does not communicate properly with the camera. Cleaning the contacts may be necessary
2. You are using a lens with an aperture ring, the ring is not locked on the maximum aperture (f22 or f32 usually)

Yeah your lens is either incorrectly attached or the aperture ring is not to the biggest number. My macro lens does this when I accidentally bump the aperture ring out of place