Then check with a simple text editor if the file looks correct. Mening that all entries are still conform to the ics spec.
If not, or if the data is lost, you have to apply your last backup.
Ulf, I checked and the file was not complete. From the time stamp for the ics entries, it seems it was doing some kind of update. I had to restore the calender from my last backup which was a week old.
Thanks for the info. D.