You must fix your mistake. Do the following:
so that means they must enter thier email in the field in order to submit?
You should find field which you re-named to something else than sender's e-mail:
CP -> E-mail Forms -> Contact Us -> Fields constructor -> Find field who's cross is grey.
There will be two fields with grey cross's, they are set by default, one of them will be
the one who requests e-mail. Simple check them out & replace, than try form yourself.
Don't forget to 'Restore default template' at 'Rebuild form template' next to Contact Us.