Can not get face time to turn on or allow me to sign in

I can not get face time to turn on or allow me to sign in. The preference section is not available when starting up Facetime, and it will not allow me to Turn facetime on either. We have 3 imacs, facetime does not work on any of them, but it works on our ipods and iphones!!!! what is going on.... does anyone have a solution. Facetime worked last week on all out. also having same problem with imessages