You could do this on your own or you can hire an attorney to help you. California Code of Civil Procedure §473(b) provides the mechanism for relief from default:
The court may, upon any terms as may be just, relieve a party or his or her legal representative from a judgment, dismissal, order, or other proceeding taken against him or her through his or her mistake, inadvertence, surprise, or excusable neglect. Application for this relief shall be accompanied by a copy of the answer or other pleading proposed to be filed therein, otherwise the application shall not be granted, and shall be made within a reasonable time, in no case exceeding six months, after the judgment, dismissal, order, or proceeding was taken. [ ] Notwithstanding any other requirements of this section, the court shall, whenever an application for relief is made no more than six months after entry of judgment, is in proper form, and is accompanied by an attorney's sworn affidavit attesting to his or her mistake, inadvertence, surprise, or neglect, vacate any (1) resulting default entered by the clerk against his or her client, and which will result in entry of a default judgment, or (2) resulting default judgment or dismissal entered against his or her client, unless the court finds that the default or dismissal was not in fact caused by the attorney's mistake, inadvertence, surprise, or neglect. The court shall, whenever relief is granted based on an attorney's affidavit of fault, direct the attorney to pay reasonable compensatory legal fees and costs to opposing counsel or parties. However, this section shall not lengthen the time within which an action shall be brought to trial pursuant to Section 583.310.
You (or your attorney if you hire one) might be able to convince the plaintiff/landlord to stipulate to set aside the default and to vacate the default judgment to obviate the necessity for making a formal motion.
Here is a link to an example of what the form looks like: http://www.courts.ca.gov/partners/documents/motvacdef.pdf
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