The HUD is an ordinary web page that opens in its own window from the Host Console. Most problems are about that window, the display or the connection.
I chose Open HUD and nothing opened
The HUD opens in a separate window. Look for a browser notice about a blocked pop-up, and allow pop-ups for the Hub. Then choose Open HUD again.
The HUD window is open but blank or black
Refresh the window. If that does not help, close it and choose Open HUD again from the Host Console. Check that your computer is online.
The picture is on my computer but not on the TV
This is a display issue, not a Gig Game one. Check your computer's display settings (extend, not mirror), the cable and the TV's input. See getting the HUD onto a TV.
The picture freezes or lags when I cast
Casting adds a delay compared with a cable. Try an HDMI cable, or move the HUD window to a second display.
I closed the HUD by accident
Closing the HUD does not end the game. Choose Open HUD again from the Host Console.