Try forcing Safari to quit and then relaunching it by pressing the home button, to see the home screen, then doucble tap the home button, hold your finger on the Safari app until it begins to wiggle, then tap the X to quit Safari, and then press the home button again. Then relanch Safari and see if it has the same problem.
If you weren't able to quit Safari, see this Apple doc -> iOS: Force an app to close
If that doesn't do it, try restarting or restoring your iPhone.