WWE issued the following statement via their website and mobile app just a short time ago.
NASHVILLE, Tenn. - WWE.com has learned that Roman Reigns has undergone emergency surgery for an incarcerated hernia. Reigns had been in Nashville to promote tomorrow's Night of Champions pay-per-view. His recovery time will be evaluated by the local surgeon and WWE physician Dr. Chris Amann, but he will be unable to compete against Seth Rollins tomorrow night. Check back with WWE.com for further updates on Reigns' condition.
The announcement went on to state that WWE Network will run a special report on this development and Roman's condition at 1PM Eastern time. We'll be back with more information as we have it, Cagesiders.
UPDATE: 1 (and 2) PM have now come and gone without any news from the Network. Presumably they are waiting for some news with which to update us. The front page on WWE.com has now been changed to say, "Check in with WWE Network all afternoon for live updates."
UPDATE PART DEUX: The WWE Network finally provided an update, saying Reigns had successful surgery and he is now recovering and resting comfortably. The WWE doctor called in to discuss the situation and said Reigns woke up with pain in his abdomen. When it became unbearable, he alerted WWE physicians and was advised to go to the hospital to get checked out. It was there that they found the incarcerated hernia, which is a medical emergency and they had to operate immediately.
This means he was working hurt for some time for it to get to this point, by the way.
It was said he's expected to miss 4-to-6 weeks but he could be out for up to three months. Dr. Amann went so far as to say Reigns' being physically fit won't make a difference because the muscle and tissue have to heal before he can do anything physical. He also said they wouldn't let him come back until he gets clearance.