Firstly, you can go on down to the customer service office or desk, and then talk to them about what is going on. Carry a copy of the ticket, so that when you complain about the show you just saw, they know what show you were in, and so that they actually believe you were sitting in the movie theatre, after having purchased a ticket for the show.
When talking to the customer service people, remain calm and avoid blowing things out of proportion. They are there to help you, so as long as you maintain your composure they are going to do just that. Now initially the person who you are talking to might not be able to help you out. This is simply because they are not designated to offer free tickets. If this is the case, ask to speak to their supervisor.
The manager is more than likely going to go down two paths. He is either going to avoid the situation and say that nothing can be done, or he will state that he is sorry for the way you were treated in your movie watching experience, and will then offer you a free ticket or a full refund. Either way, just accept what the manager says and move on with life.
Lodge a formal complain to the corporate office of the cinema company, so that they may follow up on your complaint and then go on to reimburse or give you a new ticket in the future.