Corel (like other programs) positions toolbars relative to the upper left corner of the screen. It keeps track of how many pixels a toolbar is from there. When you change monitor resolution from 1024x768 to 800x600, each pixel gets much larger and so the distance in inches goes way up and the toolbar falls off the edge of the screen.

In other words, it's still there, you just can't get at it.

To get it back oncscreen, you need to temporarily go back to 1024x768, move the toolbar near the upper left corner of the screen, and then go back to 800x600.