What Am I doing wrong with the 50mm 1.8 lens?

Outside yesterday it showed EE or EEF I think. Shutter wouldn't budge. Fiddled around with aperture etc, then worked perfect. Nikon 50mm 1.8, D AF. Yes D90 is on AF. Instruction sheet I found poor. It only shows how to connect it to camera. It connects just like lenses for film cameras 35 yrs ago.
Help.
Thank you.

Added (1). I don't want this to happen again.

Added (2). Thanks to photoace I now know what I'm doing wrong. Never thought about looking in camera's manual to find out anything about lens.

Look on page 255 of your user manual.

You must lock the lens at its smallest lens aperture

Also look on page 59 which talks about the auto/manual switch on the lens itself

The error "Fee" means the aperture is not set to the smallest opening in the lens. This is necessary for the camera to take over the setting of the lens aperture.

Of course, you could always put the camera into full manual and you would not get that error, but then you would have to use the aperture ring to change the aperture rather than from the camera's dial control.