Make sure the app is active then press Command (⌘) H then go to the Dock and click the Photoshop icon, when the window re-appears it should be fully viewable.
Otherwise, depending on your Mac OS X version you can resize the window by moving your mouse cursor to any point of the window border where the cursor should change showing you that you can drag the window inwards OR move the mouse cursor to any corner of the window & drag inwards.
If you click & hold on the window menubar you can move the entire window.
Hope this sorts your issue out for you.