Do you have two monitors? could the file dialog been dragged to the second monitor some time and leaved there?
If so, you can just drag it back from the second monitor. Or, you can delete the following registry key to restore all dialog positions to their default locations. Please find a friend who is familiar with the registry operation to help you if you don't know how to delete it.
You are going to edit a submitted issue. This issue will be changed to private draft mode until you click submit again. Are you sure?