Why is my iCal is inoperable after installing Mountain Lion?

Since installing Mountain Lion, iCal has been inoperable. I can open iCal but immediately get the spinning beachball when I do anything, so the calendar is useless; the only option I have is to FORCE QUIT. The iCloud web calendar shuts down when I try to update my calendar there, as well. Fortunately, I have my iPhone calendar, which is syncing properly, but it does not allow me to do custom repeating appointments, which are important.

Since iCal is a tool I use multiple times each day, this is a huge problem. I have searched the forum discussions but found no appropriate solutions.

Help?