I have a D90 Nikon, I mounted my Nikkor 50mm 1.8d

Sorry, but the D40x lacks an autofocus motor, depending on the lens to have one. Nikon calls such lenses AF-S. This particular lens does not have such a motor, relying on the camera to have one. The only 50mm lens currently available with an autofocus motor is this one:
http://www.nikonusa.com/Nikon-Products/Product/Camera-Lenses/2180/AF-S-NIKKOR-50mm-f%252F1.4G.html

Earlier this week the Nikon web site listed a new 50mm F1.8 lens with an autofocus motor, but then it was removed. Hopefully they will officially announce this lens soon.

One of the best portrait lenses for the Nikon D90 is the Nikkor 50mm f/1.8 lens which can be found here: http://www.amazon.com/Nikon-50mm-Nikkor-Digital-Cameras/dp/B00005LEN4/ref=sr_1_1?ie=UTF8&qid=1298955171&sr=8-1. At low f-stops the lens blurs the background beautifully and will really make the pictures of the kids pop. An added plus is that it can take great photographs in low-light situations. It only costs $125 as well.

Try cleaning the contacts (brass knobs) on the base of the lens and within the camera body (brass protrusions) very gently with a clean, dry t-shirt.

This particular lens is known to be a tight fit, especially when new. Most likely you just need to turn the lens a hair more and a little bit harder than you normally would in order to have it "click" home.

You need to turn the aperture ring to its smallest aperture (f/22) and lock it using the little orange slide. Control the aperture from the camera, just as you do with the kit lens.

Your motor may have gone bad. Try sending it to PhotoTech Repair Service in NYC. They are Nikon authorized, and can do the repair under warranty if you still have it. Also if you join they're facebook page, they will give you a 10% discount.
Here is the facebook page link, it has everything you need to send it in.
http://www.facebook.com/pages/New-York-NY/PhotoTech-Repair-Service-Inc/102527215194

Yes, you won't get 3D matrix metering or TTL distance
though. Check here for more info:

http://www.nikonians.org/nikon/slr-lens.html

If the lens has an aperture ring, make sure it is set on the largest f number. Make sure the lens is mounted fully on camera. Oherwise an AF 50mmm f1.8 lens should work on a D70 with no problem

Hello,
The Nikon D90 works well with the Nikon AF 50mm lenses and most other brand Nikon fit AF lenses. If they are not AF you will lose autofocus and metering.
Do you know that on the D90 the lens becomes a 75mm but the aperture stays the same, so for instance the 50mm f/1.8 is a 75mm f/1.8.

Hello,
The 50mm lens is fairly old and uses a different system to the later lenses. On the D90 and other later cameras you should rotate the ring to 22 and lock it there using the lock just above the aperture ring.
To adjust the aperture you use the front dial by the on/of switch.

Try it out in A mode. Rotate the mode dial on the top left of the camera to A (Aperture Priority mode) then lightly press the shutter button to turn the meter on. Then rotate the front dial and you will see the aperture reading changing.

The older Nikons changed their apertures using the ring but the later Nikons use the camera controls.