Do I really lose all my memories with amnesia?

The 2004 movie 50 First Dates, starring Drew Barrymore and Adam Sandler, features amnesia as an essential part of the plot, where Barrymore’s character, Lucy, can only retain her memories for one day.

I watch plenty of TV shows, and there is sometimes a plot where a character is in an accident, then completely loses his or her memory. Is there really such a condition?

Yes. It is called amnesia.