The Nikon D40 uses the EN-EL9 7.4 Volt Li-ion Battery. You should be able to read 7.4 V DC between the (+) & (-) terminals of the battery after it has been in the charger for several hours. If it does not, it indicates a bad battery, charger or both. There should over 7.4 V DC on the terminals of the charger if there isn't, it suggests a bad charger. If the battery voltage is OK, make sure it is inserted correctly. If still having trouble, it may be related to the camera power switch or internal circuitry. This will require returning to Nikon for repair. Good luck!
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