Are Denver and Chio Pleo RBs or the older Ugobe/Innvo models? I'm assuming they are RBs becuase of the power LED.
One possibility may just be the battery contacts inside Denver, since he appears to be getting power with the USB connection. Look inside the battery compartment of both Denver and Chio and compare. Is one of the springs bent, broken, or missing? Any rust, dirt, or anything blocking them from making contact with the battery terminals?
A second possibility may be a software problem. Load a skit onto an SD card, put the card in Denver, then try turning Denver on. Does he respond at all?
Also, if they are Pleo RBs, check the foot battery. If it is dead or has come loose, that will keep your pleo from working.