My guess…. after the car accident, she was brought to hospital… where she was mixed up with someone else, who did die… So everyone was told she died and was buried etc…

Meanwhile, the real Sydney, who remained unidentified at the Hospital, lay in a coma, woke up one day with no memories etc… cut to 10 years later and she just happens to have moved into an apartment on Melrose…

However, knowing Sydney, she probably faked her death for some insurance scam – or to get away from the rat Craig!