When I turned my camera on I had an message "Error. Press shutter release button again." I have tried pushing the shutter hundreds of times and am not getting anywhere with solving the problem.

Try switching the camera on and while it is on remove the battery. Leave it for 30 secs and replace the battery then switch it of and then on. This process sometime clears Error Codes.

It is nothing to lose to try this:
1. Remove the battery (the charged and good working battery),
2. Switch the power to ON position,
3. Press the shutter button for 5 seconds.
4. Switch the power to OFF position,
5. Insert the battery,
6. Switch the power to ON position,
7. Try to take a picture.

Please post the result here.

Check the batteries

Turn the camera on and look into the view finder while you try to press the shutter button. Check the information in the view finder to see whether you are getting an error message. If you see an error message in the view finder, remove the lens cap and proceed to take a photograph but, instead of pressing the shutter button this time, press the button near the top of the camera on the back which says "AF/AE. If the camera takes the photo it means you have somehow initiated the auto focus/auto exposure lock which the camera will not override when attempting to take a photo using the shutter button. When you use the AF/AE button, you override this system. To remedy this issue, when you get the error message try pressing the shutter button again. Also, remove the lens to see if the shutter inside has frozen in the "locked" position. If you find this to be the case you will need to contact Nikon Customer Support. The link for Nikon Support is below. The numbers are half-way down the page.

http://support.nikonusa.com/app/answers/detail/a_id/238

If you can dare servicing youself You can open bottom cover of and turn a little red wheel manually. try pressing the release button now and will start working. If you have this problem re-accuring, then get it checked wih professional.

try this:

http://jacko0.wordpress.com/2009/10/13/d40x-underside/

you might have a stuck shutter. have it checked by nikon..

The manual (pg 112) simply, and unfortunately, states this message "Press shutter release button again. If error persists, consult with Nikon-authorized service representative".

Some questions that might help: have you had the camera in some extreme conditions, damp, sand, etc. Clean the shutter button really well. Maybe use a small air blower and clean in the shutter release creases.

In the end, if the error continues, it will need to be serviced.

I was taking pictures, then when I pressed the shutter release button, it took a long time. Then the error message came up. I tried pressing the shutter release button again, nothing happens. Still the error message. Please help me

That may be the best advice :
http://nikon.com/