Hello and welcome aboard
If there are multiple windows open, one is active and the others are inactive. Even if it is a single window opened, but desktop or taskbar is clicked, then desktop is the active window, and the other open window is inactive.
In order to access the inactive window, you must to make it active at first, and then only you can use that window. To activate it, you are to mouse click on it.
Inactive windows look a bit faded, in fact
But if the situation is ... an window is open, say for 5 minutes, the computer is untouched in that time, and the window got inactive automatically ; then it is an issue. is it your case, let us know.