Problems with PowerConnect 3524p

I have 4 PowerConnect 3524P switches. All 4 were purchased, configured and installed at the same time. Units A & C are installed and working perfectly. Units B & D were recovered from other sight locations that were closed and were working fine when they were shut down. I connected to Unit B via the console port using Putty and a null modem cable and was able change the stacking ID's just fine. After an IP address change, it will be added in a stack with Unit A. Unit D was to be added in a stack with Unit C, but I'm having problems with it. Using the same cable and Putty setup that I used to connect to Unit B, I get no response from the switch, and nothing shows on the Putty screen but a green block in the upper right corner. When I unplug power, wait about 10 seconds and restore power and try to change modes to stacking (within 15 seconds), nothing happens. No stacking LED ever starts flashing and the stacking button seems not to work. I did connect a laptop (on the same subnet as the switch) to one of the ports and get link and activity lights but nothing else. I know what the switch's IP address is supposed to be, but cannot access it via the web interface. Any ideas on how to resolve the issue, or is this unit dead?