First of all: Check your Files icon in the launcher (on your left for most systems). There will be a little progress bar over if it if it's still copying something.
In that case, if you're lucky, you can restore it in tree ways:
Look for a file copy icon in your system tray (upper right in most systems). If there's one, click it and the file transfer dialog will reopen
Right click on the Files icon in the launcher (yes, the one with progress bar on it) and choose "show copy window"
On your Files icon in your system tray (the one with a progress bar) are some small arrows in its left, one for each window. If you manage to click in the arrow, the corresponding window will open. Your copy window might be one of those
In the case none of this work for you, you might be seeing this bug