Subscribed iCalendar items suddenly stopped appearing in destination calendar

I use an online calendar (AirSet.com) and have a subscription to my appointments using the I Want Sandy iCal link. This has worked perfectly until just a few days ago, when multiple refreshes had no effect. Everything on the I Want Sandy website looks good, but nothing is showing up on AirSet. Any suggestions? I know nothing about how .ics works, so I don't know what to try first.