Hmm. I don't think it should be touching your calendar but go to Options > Status - what's the File Free?
File free is 11642584
The only thing that I did was add a personal ringtone and I noticed that when I selected to delete it from the media library, it still stayed there. So i just popped out my battery and when i turned it back on, the ringtone that I deleted no longer showed up.
But other than that, I really don't understand how my calendar was wiped out. I depend on my calendar!!! =(
Set how long your device stores calendar entries
1. In the calendar, press the Menu key.
2. Click Options.
3. Set the Keep Appointments field.
4. Press the Menu key.
5. Click Save.
Your BlackBerry� device deletes calendar entries that are older than the number of days you set.
To restore calendar entries, set the Keep Appointments field to a longer period of time. Calendar entries that you received on your BlackBerry device within the set period of time appear in the calendar ...
Found this in my Users Manual. The default setting is 60 days, you can choose Forever.