Nothing happens when I turn the camera on.

Top cover needs to be removed to fix it.

try a web search, you'll be surprised what you may find. just enter 'error 7 Nikon N90s'

when you place the film in the camera & close the door does the film advance ?

You may download a copy of the manual from http://butkus.org/chinon/nikon/nikon_n90s_af/nikon_n90s_af.htm

FEE error message displayed on SLR camera LCD Description If the cameras LCD screen displays "FEE" in the area where the aperture is normally displayed, please ensure that the lens is attached to the camera correctly. This error message will appear if the lens is not attached correctly to the camera or if you are not using a CPU lens such as the Nikon AF or Nikkor P lenses. Alternatively, if the lens has an aperture ring, it may not be set to f22 (or the highest aperture number on the aperture ring - normally marked in orange). Select the aperture number marked in orange on the lens and lock it using the aperture locking switch to prevent this occurring again.

The FEE error message

Set to F22, or the highest aperture number - normally marked in orange. Note the locking switch.
If the above did not resolve the problem and you still experence the the error display FEE, please contact your local Nikon office. Before doing so please also check the following.

That the EE servo coupling highlighted above on the lenses has not been broken. That the F-min switch highlighted above on the camera body has not been broken. If either of the two items above are broken please contact your local Nikon service center.

If you are paying more than 50$ for the shutter unit, it probably is not worth the time. I looked at what used N90s bodies sell for on eBay, and most sell for under 100$.

The job of replacing the shutter really only requires 3 tools. A cross point screwdriver, (with a metric tip rather than the US sizes), a pair of tweezers and a soldering iron. An anti-static mat for electronics work is also advisable.

You will be removing all of the covers on the camera, then several interal screws,plates,pressure connectors with some de-soldering on bridge connections to separate the circuitry. Most of the screws are different lengths and sizes, and need to be put back in the proper holes. The circuitry is very delicate and can tear easily with a wrong move.

If this sounds like something you would like to attempt, I can elaborate further. I've replaced a few shutters in the N90s. A few took about an hour, but not always.

The film transport functions on the N90s involve a series of small gears in the bottom of the camera. There may be a damaged gear or possible debris blocking the movement. Replacing any of those components is not really a do it yourself unless you are comfortable working with small screws, about 20 of them. I can guide you further if you are interested

If the switches and contacts are dirty that would do it. If that happens the cost of service may exced the cost of buying a used one.

I do know that on earlier models like the N90s and 8008s, the small nut that holds the battery clip in place activated a cut off switch. If the switch was faulty, the camera would not turn on. There may be a similar switch in the F100.