I had the same problem, and JUST figured it out. First of all, if you saved your iCal data as an iCal archive, it won't work with iCloud syncing. However, it will get you to a solution. Follow these steps and everything should work:
1. Turn off any Internet connection. If you don't, iCloud will keep deleting it.
2. Restore your iCal data with the iCal archive you'd saved.
3. Next, for each calendar you have, select it, then select File>Export>Export... and save each calendar you want to sync back to iCloud individually. If you want to restore tasks, you have to do the same with each task list. It seems you have to have different names for task lists and calendars, so make sure you know which one is which.
5. Turn on your Internet connection again. Go to iCloud.com to see what's on the calendar.
4. If you have any data left in iCloud, I would delete it to start fresh. After this is done, make sure you have created the calendars and task lists you want.
5. Start up iCal. When it syncs with iCloud, everything will disappear again, but this is ok. Make sure you have the same calendars and task lists as are on iCloud.com.
6. Double-click on each saved calendar and task list .ics files and match them to their respective calendars and task lists that you want to sync. iCal should now import and upload the data to iCloud correctly. You don't have to wait for each calendar to fully upload--you can do them sequentially and they'll all upload sequentially.
Hopefully this should work for you. If you are using a third party task manager as I was (which somehow caused the duplicates as far as I know), I ended up deleting and reinstalling the app and resetting the sync from scratch, getting all the data from iCloud. With a little tweaking, everything is back to normal. Whew!
Hope this helps.
I can see that what you describe is working as my calendar is repopulating. However, it's only with a few entries which are a small subset of all the calendar entries I'm trying to restore. As a check, I looked at the .ics files that were exported and all were the same small size, around 39K. Is there some trick necessary to exporting all the entries for a particular calendar before performing the File > Export > Export? There must be because I'm only getting a small sample in my exported .ics files.
I later learned that simply checking the Calendar box is not enough to export the entire calendar. You must select to highlight the calendar in the iCal Calendars dropdown. Once this is done then the entire calendar's contents will be exported. Otherwise only a specific calendar event is exported. I'm back in business so, again, thanks John!
Thank you. I got most of my calendars back using your advice. Still can't quite figure out what I did to delete everything.
Good thing my laptop was turned off when everything disappeared. I unplugged the modem and was able to backup the iCal on that laptop and then overwrite the iCloud calendar as suggested.
A huge THANK YOU John - this solved my problem
Shame that deleting a calendar is SO easy - an 'are you sure' would be a reassuring delay. I am sure I acquired a few grey hairs running aounrd my house trying to stop different Apple devices updating from the Cloud and deleting one of our calendars (my husband's . . . .). I caught my Macbook just in time.