PC-HELP.CZ

Make sure you have the Steam Community in-game overlay enabled in Steam's settings. To check, go to:

Steam > Settings > In-game tab.
Check the box next to Enable Steam Community In-Game.

Additionally, each Steam game has the option to remove the In-Game Overlay. Please check your individual games as outlined:

Right click the game in your Library
Select Properties
On the General tab, check the box next to Enable Steam Community In-Game

The default shortcut to open the Steam Community while in game is Shift + Tab. You can change the shortcut in by clicking Steam > Settings > In-Game.