Why the selection tool (rectangular marquee, lasso, etc.) is not working / nothing is selected?

If you cannot use any selection tool (rectangular marquee, circular marquee, lasso, magic wand) you might have chosen by mistake a special selection mode that makes all selection tools “stop working” (or at least working differently as expected by default):

41 34
If you have those buttons selected, you can select the first one which is the default mode and the selection tools should start working again as expected:

48

Related to Selection marquee (M)