First of all, don’t panic and contact the university letting them know that your English proficiency is not good enough yet. Post that, re-take the exams as soon as possible. Identify your weaknesses and work on them while preparing for the test. A lot of programs may be available in your home country, the US or online that will help you improve your English proficiency. If everything else fails, consider taking a transitional course before transitioning into an undergraduate program.