The surface memory associated with a DirectDrawSurface object may be freed, while the DirectDrawSurface objects representing these pieces of surface memory are not necessarily released. When a DirectDrawSurface object loses its surface memory, many methods will return DDERR_SURFACELOST and perform no other function.
Surfaces can be lost because the display card mode was changed or because an application received exclusive access to the display card and freed all of the surface memory currently allocated on the card. The IDirectDrawSurface::Restore method recreates these lost surfaces and reconnects them to their DirectDrawSurface objects.