Determine if its the game board or monitor.
Turn up the SCREEN control (on the flyback transformer on the monitor under the Focus dial) and see if you get a white screen. If so, the monitor is perfectly fine, it's your game board that is defective or wiring between the gameboard and monitor. Adjust the blue gun DRIVE and CUTOFF controls and see if it makes any difference.
If it's your monitor it could be a number of items... Bad connector, bad transistor or IC in the circuit, disconnected cable between chassis and neck board, bad potentiometer controlling the blue gun, bad socket to the tube, broken pin on the tube, or dead gun in the tube, or broken solder joints on the monitor chassis.