If you file a complaint for Unlawful Detainer and your tenant does not answer within five court days, you can take their default and get a writ of possession against them.  Or, if they answer and do not appear at trial, then you can also take their default.

However, tenants have the ability to request that the default be set aside under California Code of Civil Procedure 473 in instances where there is mistake, inadvertence, or excusable neglect.

If the writ has been issued you must act immediately and file an ex parte motion to set-aside the default judgment.  The rules for ex parte motions are different from county to county.

In Sonoma County, for example, you can go in and file your papers Monday through Friday but it must be filed before 8:30am.  You must give the other side “reasonable” notice, ideally at least by 10am the day before. 

Although the issuance of a writ is not a good thing, it is possible to overturn the judgment if you have a good reason for failing to respond or show up in court.