It'll hurt you, but I don't think it'll ruin you and make every school automatically reject your application. Depending on your GPA, GRE, and the rest of your application you may not have to spend the time to retake that course. Retaking the course and getting an A, however, will look good as well as quell your worries about your chances. If I had the chance to retake some of the Cs I got, I would totally do it to know I have the best chances I could have gotten for the application process.