If you have a dual monitor setup, sometimes a game started through Steam might move to a secondary monitor. This is a problem with specific games, for example Fallout 4 after alt-tabbing.
Solutions commonly suggested for Steam games not starting on the primary monitor are not relevant to this problem. These include changing the game's configuration files, using the Windows key + SHIFT + left arrow key combo, or temporarily setting the game to windowless and dragging it to the other screen.
What does work is:
- Turn off the secondary monitor
- Wait for the game to move to your primary monitor
- Turn the secondary monitor back on
The game will stay on the primary monitor including the next time you start the game.