How do I copy a calendar from one iCloud account to another?
If it appeared to hang (rather than throwing an error) how long did you let it run?  I found that iCloud took rather longer to import a .ics file of only a few hundred KB than my available raw upload speed would suggest: 1-2 orders of magnitude longer!

If it really won't complete, try importing .ics file into a local Mac calendar first - it may not work any better but it's faster if your calendar is large; and just maybe iCal will be able clean up the calendar or continue past a bad entry that stops iCloud.  

If that doesn't work (and you're patient!) you can try exporting &amp; deleting, then importing to another calendar, half your events, then half the remainder, etc., til one segment fails to import. Now that you know which half contains the error, export, then import, the other half of the remaining events.  Now repeat the halving process with the events left in the source calendar.  When you get down to only a few remaining events (that must include the bad one), enter the rest by hand.
Now export that reconstructed calendar and upload it iCloud.