Welcome to the discussions,
you can replace the calendar info on the phone with the one on your Mac during a following sync by choosing this option from the info pane, advanced section, with your phone connected.
If the contacts on your Mac are damaged as well, the only other option would be restoring the phone from a former backup and syncing to the Mac afterwards, or using a backup of your Mac to restore the calendar info, in that case it would be good if you were using Time Machine to back up your Mac.
iphone 4.2.1 update nuked my contacts
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