Ok, sorry. I suppose your unasked questions are, "Why?" and "How do I fix that?"
No one is going to be able to answer either question until you provide more information than, "It doesn't work."
What is your OS version?
What happens when you try to open iCal?
(Editing is obviously not possible if you can't open, so that part is superfluous.)
Did you install something recently?
...and any other detail you can think of, even if you don't think it's related. We can sift out extraneous information, but cannot make stuff up that is likely to help.