Game starts, mouse cursor moves, but can't click & use keyboard.

When i start the game (via steam), i can only see the menu, then I can do nothing, nothing reacts to click or key hits.
Only Alt-F4 closes the game.
Even steam Shift-tab doesn't bring the steam interface.

Anyone has that too ?

PS : this may not be specific to this game, as I have this issue with some other games, but could'nt find a solution.