Find out the reason the class was cancelled. The most common reasons are low enrollment, lack of faculty, controversy and lack of funding. The best way to find this information is to ask the faculty member who was scheduled to teach the course. You might also ask the department chair, or your academic adviser. If these people cannot, or will not, tell you why the course was cancelled, make an appointment to speak with a dean or president at the college.
Determine what will be necessary for the college to reinstate the course. If the course was cancelled due to enrollment, you might have a chance to get it restored, if you recruit students. Start a petition of students who will register for the course, if it is offered.
If the class was cancelled due to lack of faculty, your only hope is to convince a faculty member to teach the course. This is basically asking a professor to volunteer to work overtime.
If the class is cancelled due to lack of funding, you can always offer to pay for the course. One college in California whose budget was cut, is offering to restore cancelled courses for $6000 each. If the class was cancelled due to controversy, try starting a petition of students and citizens who object to the course being cancelled.
Talk to the department chair or dean. Politely explain your position, and ask what has to be done to reinstate the course. Offer to show them petitions or other documentation that might help your case. If they tell you that there is no way to reinstate the course, thank them for their time and make an appointment with their superior. Remember, the fight is not over until you admit defeat, or the semester begins.
Discuss alternatives with your adviser. If you need the class to complete your degree, ask if another class could be used instead. Check around to see if another local college offers a similar course.