You may have put the window on a different workspace, or it might be hidden behind another window. You can find it again by using the activities overview.
Open the activities overview and make sure the
If it is not displayed here, it might be on a different workspace. Click on the different workspaces (in the list on the right-hand side of the screen) to try to find your window.
You can also right-click on the application in the dash. All open windows of an application will be listed. You can click on the window you wish to switch to in the list.
A faster method of finding a window is to use the
If an application has multiple open windows, there is an arrow below the application icon which is seen when tabbing through the apps. Pressing the down arrow on your keyboard will allow you to tab through the multiple windows.