Nikon n90s instruction manual - Nikon N90S 35mm SLR Camera

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 ?

I believe you mean the focusing screen. While this is an interchangeable part on higher-end Nikon cameras, it's not on the N90s. Your local authorized Nikon repair center may be able to replace it, if they can get a spare.
Before repair, bear in mind that the focusing screen will not directly affect any of your pictures. Due to the very nature of a SLR, when you take a picture the mirror flips up next to the focusing screen to let the light from the lens through to the film, bypassing the focusing screen and the rest of the optical viewfinder system. If the scratch isn't very big then while annoying it won't affect camera operation. If the scratch is very large, on the other hand, it may affect the exposure metering and, of course, your viewing.

Have you tried replacing the batteries? It sounds like the batteries may just be too low to wind film, but can function without a load.

I can't remember if the N90S has the vertical grip release or not, but if it does, have you attempted to use it, to see if the camera functions from that release, or on fully manual settings with a cable release? Are there any error messages reading out on the LCD top panel or in the viewfinder? Are you using an attached flash? What settings are you using?

If it's not functioning, on any settings including manual with film loaded, then it's most lilely in need of a visit to a repair shop to figure out where the issue lies.

But, I'd try new batteries first.

You can download a complete manual from
http://butkus.org/chinon/nikon/nikon_n90s_af/nikon_n90s_af.htm

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.

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.

Press down and slide one way or the other. It's generally held by two pins that slide into hole slots and lock in place