My nikon D40 says my battery is dead

If you are using the same battery, it could be it has reached the end of life. The charger can indicate the battery is fully charged, because the voltage has reached a correct level, but still the battery can have one or more bad cells. So as soon as you start using the battery, it can lose enough energy to give this nasty message. If you put in another battery and still have the same message, there must be a serious problem in the camera.
To check your battery, just after the camera has given this message, with a multi meter. You will see the voltage has dropped at least 1,2 volt beneath the voltage printed on the battery. The typical 7 volt Nikon battery, will give 8 volt on the multi meter. But placed in the camera it will drop to 7 volt. If the multi meter reading is less then 7 volt it has at least one bad cell and should be replaced.

try another new battery, maybe your battery is dead.

sometime if the battery is dead it still shown fully charge on the charger, try change another battery or with new one.

Follow the instructions in this blog post to fix your problem.

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

A couple of things to try, and if these don't work you will probably need to have it serviced.

1. There is a reset button which may fix it. It's described on the first page of "Troubleshooting" in your manual. The camera diagrams at the front of your manual will help you find it, near the connector cover.

2. If it doesn't work after the reset button, pull the battery out and leave it out for two days, then put it back and try again.

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

Can you explain more please ?

The Nikon D40 does not have shutter release or as known cable release, but does have a wireless remote control release.

If the shutter is not working properly....

Remove the battery and reinsert.
Remove the memory card and try again
Check the lens is correctly fitted.
Is the lens a compatible fit

i was quated £250 to replace a buckled shutter unit can this be done any cheaper

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

Your battery may be near-dead. Nikon SLRs disable the shutter, but still let you view pictures, when the battery is low. Solution? Simply charge the battery.