It's merely offscreen or very small.
Practice on a window you can see then try it on Photoeditor when photoeditor is active application (it's selected on the taskbar).
Alt + Space, M, then arrow up and left (or down and right, up and right, etc)
Alt + Space, S, then the first arrpow key you press is the edge whose side is being adjusted. So if you press right arrow first the left/right arrows adjust the right hand side of the window.
Or
With only PE open right click a blank area of the taskbar and choose cascade
or
Type regedit in Start - Run, navigate to
HKEY_CURRENT_USER\Software\Microsoft\Photo Editor\3.0\Microsoft Photo Editor
and alter the numbers at
InitialPosition
The first two are the upper left corner of PE relative to the upper left corner of your desktop, the next two are the width/height of PE window. All 4 are in pixels.
So if at 800x600 resolution I'd use 30, 30, 740, 500