First thing I would do is simply power cycle the phone. Turn it completely off, then back on. If that doesn't work, I'd delete it from the home screen and then re-add it.
To remove the Widget: press and hold on the clock. A trashcan icon should appear at the top of the screen. Drag the clock widget to the trash can.
To add it back: long press on the screen where the clock was. Tap "Apps and widgets." Tap "Widgets" at the top of the screen. Scroll right until you find the clock you want, then long press on it. A light colored box will appear in the middle of your screen. Drag the clock widget into that light area, and let go.