Do you have any backups from before the upgrade? If so, you can restore your calendars from backup. If you use Time Machine for backups, quit Calendar and then choose Go To Folder from the Finder's Go menu and enter:
When the window opens, open the Time Machine app or choose Enter Time Machine from the Time Machine menu, go back in time and restore everything in that folder.
If not, without knowing exactly what happened, there's not much we can tell you. I'd advise you to make an appointment at the Genius Bar at your local Apple Store.
Thomas...that's the strange thing, too. When I went into Time Machine, the only backup available was yesterdays. I had MONTH's worth of backups but those seem to have disappeared. Also, when I search for a specific picture, for example, it's nowhere to be found. I still have the image or thumbnail in iPhoto but can't find the actual file.
I guess it's time for an appt at the Genius Bar. Thanks for your suggestions. Have a nice weekend.
If your backup volume was too small, it's quite possible that the first backup following the upgrade would have removed most or all of your older backups in order to make room for all the new or changed files installed during the upgrade. Your Time Machine backup volume should be at least 2-3 times larger than what you're backing up... is it?
In any case, though, I think visiting the Genius Bar would be a good step to take.