Can someone tell me why my function is not working?

[kbw]

Yes, well no. You don't need to pass grade in. It'd be more like:

char getGrade(double totalNumGrade)
{
    char grade;

    if (totalNumGrade >= 90.0)      grade = "A";
    else if (totalNumGrade >= 80.0) grade = "B";
    else if (totalNumGrade >= 70.0) grade = "C";
    else if (totalNumGrade >= 60.0) grade = "D";
    else grade = "F";

    return grade;
}