I just wanted to provide a solution in case anbody runs into this issue. I have a keyboard, a Unicomp Model M clone (P/N UA40P4A), that apparently draws so much power over USB that the HDMI no longer connects to any display. At first i got what can best be described as “sparkles” on the screen, and then it failed to connect entirely.
Before deciding to RMA your Nano, you might try unplugging any USB devices, even keyboards and mice. If you have alternative access to the machine (Ssh, serial) you can check whether this is indeed the cause by running “journalctl”, typing in “/gdm”, pressing enter, and looking for errors related to ACPI.