It may be just a coincidence, but after upgrading to Mountain Lion I can't open iCal — the desktop icon is only an alias, as is the program in the Applications folder.
How do I get my iCal back?
When Mountain Lion was released, iCal was transformed into "Calendar." Look for the "Calendar" application in your Application folder.
Thanks Ferd II, you da man!
You are welcome...glad to help.
Retrieving data ...