Today I wanted to do some work with Indesign and ran into a problem that existed for some time: selecting "help" starts up the Adobe Help Center (AHC) but no window is visible. I couldn't tell if the window was invisible, hidden or just plain not there, but I couldn't use it.
I searched the knowledge base of Adobe and ran into all kinds of problems. Did you know that AHC uses Opera's rendering engine? However, the only thing related to my problem (AHC runs fine just doesn't show a window) I could find was a reference to a similar problem on Windows (http://www.adobe.com/support/techdocs/331717.html).
Since they suggested to throw away an XML file (AHC_Prefs.xml), I assumed that a similar solution would work for me. The file is located in /Library/Application Support/Adobe/AdobeHelpData/.
At that location I also found a similar file (AHC_HelpDesk.xml) that had been changed today (due to numerous attempts to start AHC), so I assumed it could be thrown out too. After looking into them, I could not find anything about a location.
There is however a file called Assistance.prefs in the Preferences subfolder that has location coordinates. It finally dawned on me: last time I could use AHC I was using two displays on my laptop and the AHC window must have assumed a position that is invalid when using only one display.
I opened the file in a text editor and changed the following lines:
from a large negative value to 100. Saved the file and restarted AHC. Lo and behold, my window was back!