If you have checked the correct view is being used and if there are no
filters then check to see if you have any items in deleted that can be
restored.  Also check to see if you have any archiving setup under Tools
Options Archives - or right click on the calendar and select autoarchive.
The items may have been deleted or moved based on the archive setup. If you
are on an Exchange server and hopefully it is backed up then a restore
should be able to be done.

Regards,