Puck & Glory needs WebGL 2. Try an up-to-date Chrome, Edge, Firefox or Safari, and check that hardware acceleration is switched on in the browser's settings.
The graphics were reset (a driver update, a graphics switch, or a long time in the background). The game is paused and carries on as soon as the picture is back.
Still nothing? Reload the page. A game in progress is lost; a Union Cup run carries on from its last finished game.