Some error handlers may disable display of a pane if there is an error during the display. You can check if a pane is in this state by calling pane-can-restore-display-p, and if so you can use pane-restore-display to restore the display. That assumes that the code was fixed, so is useful only while debugging.
The Window Browser tool in the LispWorks IDE allows you to restore the display interactively using these functions.
CAPI User Guide and Reference Manual (Macintosh version) - 25 Feb 2015