Try this and reply,
Clock is linked with weather app
If you disabled the weather app. enable it and restart your phone. it should work
Settings> Apps> Disabled Section(Slide to move)> Weather> Enable
Use other clocks from widgets
Tap on app tray(6 dots at home screen)>Widgets>Choose clock if any
Or
try some third party apps like AcDisplay(Active display app) which will enable clock in lock screen
If you are on kitkat then you can use third party app widgets on lock screen else you can't. If you are on lollipop there is no other way than to renable weather app... The widget disappears after removing weather app because the widget on lock screen is actually of weather app rather than clock app. Thanks