Undoing calendar sync between iPhone and Outlook
You may be able to restore your Calendar from a previous back-up in iTunes.
Check the time and date of the previous back-ups.
Windows: Edit > Preferences > Devices tab
Mac: iTunes > Preferences > Devices tab

Restore from your back-up and see if your events come back. Restore by right-clicking (Option+click on a Mac) on the device and selecting "Restore from back-up"

The best way to sync with Outlook would be to download the iCloud Control Panel. This lowers your risk of losing Calendar data, as in your case. Of course, you need to have iCloud enabled on your device as well. This requires iOS 5 or later.
You can check this under: Settings > iCloud
You can download the iCloud Control Panel here: http://apple.com/icloud/setup

Make sure that when it is installed and you are signing in, that you use the same AppleID that is used with iCloud on your device.