Can't find my "Find in Files" window in Visual Studio 2015

Activate it (Ctrl+Shift+F), then with the mouse select Window->Dock. It docked it above my Output window and made it visible again.

Weirdly, when I did Window->Reset Window Layout, it reset everything, but did not show that window or make it show up when I chose Find in Files.

Window->Reset Window Layout should revert to the default layout.

If you have exported your settings previously (before you lost the Find in Files window), you can re-import those settings as well. That's found under the Tools menu. You can clear 'All Settings' and select General Settings->Window Layouts only, if you want.

If you have multiple monitors check if it opened there. Especially if you have your laptop lid closed. The monitor could still be active.