Often times, this may be a simple fault. Have you tried turning the iPad off and on? Please, do the following: Hold down the Home key on the front of the iPad and the power button at the same time. Hold both of the buttons down until it resets and then let go.
If the accounts do not return, you should be able to simply add the iCloud account to your email app. Choose to add a new account, choose iCloud, and follow the instructions.