I've seen more reports of problems with the Yahoo accounts than any other on this forum, and much of the time it's a server issue that fixes itself; no announcements, no apologies. So don't be surprised if it just starts working again as suddenly as it stopped.
The Yahoo account does not support groups, so if you don't have more than one account syncing to the iPad (or a local contact database) the red groups ribbon will not appear.
Here are some things you can try.
On the Yahoo site, make sure all your contacts are showing up in the All Contacts list. This is the only list that will sync.
Try turning your Yahoo contact syncing off (delete from the iPad) then back on again. Open the app in between to make sure the contacts get purged. {I had to do this the other day to get my Yahoo contacts to re-appear.}
More extreem you can try deleting and re-adding the Yahoo account. This is fairly safe, but it's a good idea to check out the Yahoo site first and make sure all you contacts and events are there before letting them be deleted from the iPad.
Good luck.