|And as far as you know the charger is OK too?|
You can make sure the battery contacts in the camera aren't bent or discolored. If they look bad you can polish them up with a pencil eraser. Don't let the eraser remnants fall into the camera.
If that's not it, it must be either the on/off button or some other internal failure which probably means a shop will need to fix it.
Is it still under warranty? If so, don't waste too much time on it and risk voiding the warranty. Just contact Hitachi and make arrangements to send it back.