DIAGNOSIS The setup is hung trying to download a firmware update over your Wi-Fi. This is a common network handshake issue during initial setup.
FIX You need to force a restart and potentially use a different network path. First, unplug the Hub from power. While it's off, restart your home router by unplugging it for 30 seconds and plugging it back in. Once your Wi-Fi is fully back online, plug the Nest Hub in again. When it boots to the setup screen, use your phone's mobile hotspot (from a different phone if possible) to complete the setup and update. After it's fully updated, you can then switch it to your home Wi-Fi in the device settings.
IF THAT DIDN'T WORK If the hotspot trick fails, there's a factory reset pinhole on the back. Straighten a paperclip, press and hold the button inside for about 15 seconds until you hear the confirmation sound. This wipes it completely. Start the setup over from scratch, and it will often pull the update from a different server.
HEADS UP Never interrupt it or unplug it during an update once it starts progressing, you can brick the device. Let it run its course once it gets moving.