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It is possible to make a "motion to set aside default" if you were not properly served with the original summons and complaint.
The California Courts have a helpful self-help site that guides you through the process: http://www.courts.ca.gov/12726.htm (note: the site gives a deadline of 180 days from entry of judgment if you were improperly served, while 6 months is the general guideline, if you can prove fraudulent service, or some other extraordinary circumstance, you can sometimes try filing this motion beyond that time - but I highly suggest retaining a lawyer to do this for you).
Setting aside the default will allow you to file an answer and defend the matter on its merits. (I do not have the details of the case and we cannot provide you with legal counsel - if the judgment being sought here is significant (which it very well could be given the subject matter), I would again suggest retaining a lawyer).