iPad 2 lost all Contacts, Calendars.
The data is still in iCloud and my iMac has all this information
I've tried Wi-Fi, iCloud, and syncing via iTunes, but can't retrieve information.
Any one of the paths - WiFi syncing down from iCloud or choosing to sync contacts over iTunes should work to get contacts on your iPad.
Since it's the one device having problems, you will want to back it up and restore it to factory settings and test one set of data - perhaps contacts from iCloud.
At the end of the steps linked above - choose to set up as new and then text contacts. This isn't a definite answer since you'll need to troubleshoot a bit, but it's a start on that process.