Hi RM,
I'm not an expert in this area, but I will try. I noticed that when I tried to start Xcode that the Welcome Window woud load for a split second; it would then be replaced by a work window or file window which would not populate with any data.
This indicated to me that the application was looking for something it could not find. I looked for all files associated with Xcode using an application called EasyFind. I looked for any file that was created or altered during the time I tried to open Xcode.
From there, I tried to use the information gathered in the previous step to identify paths to preferences, caches, temporary work files and other similar files. Mostly, I looked in the USER Library which can be accessed from the GO menu while holding the OPTION key. After I scoured the USER Library for transient xcode files, those changed after colsing the application, I also scoured the root level Library for similar files.
After deleting all of the files that I found, the application started normally. Probably the mosy important files are the preferences and the cache files because they might point to any project you might have started.
As a lowly novice in programing, I probably started a few project files; never did much with them, and they might have been moved ot corrupted.
I hope this helps; if it does'nt just contact me again.
Vincent