No on/off switch for iMessage in settings

When I attempt to log into iMessage on the ipad I get a message

Verification Failed. When I go to Messages in Settings on my iPad there is

no on/off button, so I can't turn it on or off. One of the ways recommended to reset it is to

switch iMessage off and on again, but I can't because no button is

shown. IMessages had been working fine until today. And FaceTime is stll working fine, but also does not have an on/off button in Settings.