Normally if you select some text in the Visual Studio editor and press Ctrl + F, when the Find and Replace modeless dialog appears, your selected text is automatically shown in the “Find what:” textbox/dropdown.
For some strange reason this has stopped working – is there a setting that controls this?
It did not stop doing that, and you did not tell it yet to use the selected text: Go to Tools → Options → Find and Replace, and check the “Automatically populate Find…” option.