You swapped from the output screen to the QuickC screen,
and QuickC does not have enough memory to restore the
output screen. If you try to swap back to the output
screen (F4), the output screen will be blank.
 
If you want to view the output screen, exit QuickC and
remove any memory-resident programs to free up memory.
If this doesn't work, try opening the Output window on
the QuickC screen to view output.