Control your emotions. Control the situation by starting to control your emotion. Normally, when you are blamed for something you know you do not deserve, negative reaction takes place. We tend to become irrational and makes impulsive actions.
Research your data first to support your stand. If you believe that you are not the person to blame, gather first the pertinent data to support your stand.
Document your response to the issue. A verbal response will not do good in response to the issue. Most of the office nowadays use e-mail for inter-office communication, creating an e-mail will do and copy furnish your immediate superior if necessary. Your email should be clear and will only state about the issue. Write a short introduction then enumerate your points to prove your stand.
Escalate the issue through the proper channel. An office environment will always have the chain of command. If the issue was not resolved with the involved person, escalate the problem with your immediate boss. A personal meeting is preferred to discuss the matter better.
Do not take the issue personally. Any issue related to work should not be taken personally. Unless you are being harassed in the office, a single work related issue should not affect your personal well being. Move on and focus in office productivity.
Handling conflicts and blame issues in the office is a skill, there will always be other issues but what is important is we know how to deal with them effectively and we rose victorious from it.