I've had similar problems. What seems to happen is when I reboot one of my pc's, I can't get into the nas. What I have to do is double click on the drive letter that I had mapped to this device, which it then asks me to input the password for the user account I set up, then it works fine. Each time I reboot, I have to do this in order to access any of the data on the drive. It doesn't cache the authentication session for whatever reason. Maybe that is what is happening with you too?
First, if you have set a static IP, try to ping the NAS and if you get a response, try to connect a drive letter using the d-link utility.