Can my house be foreclosed without my knowledge?

All foreclosures require lots of notice. Since you said, you got a call from the city, was it a tax foreclosure? You will have to find out what happened and try to reverse the foreclosure if you did not receive notice.

Under California law, a foreclosure must be commenced by recording and serving on the property owner of a Notice of Default. If the default is not cured within 90 days, the lender must then record and serve a Notice of Trustees Sale. I have heard stories of homes that were sold at a trustee's sale without any prior notice. If the lender failed to strictly follow the statutory rules for notice, the homeowner can file a lawsuit for wrongful foreclosure and regain title to the property. You will definitely need an attorney for this. Please take steps immediately or you may lose your rights.

You were supposed to have received the notice of the default and notice of the trustee sale. Unless you do not live in the property, it would be difficult to argue to court you were not aware. Generally the notice of trustees sale is photographed on your property after it is posted.