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 return DDERR_SURFACELOST and perform no other action.
Surfaces can be lost because the display card mode was changed or because another application received exclusive access to the display card and freed all of the surface memory currently allocated on the card. The IDirectDrawSurface3::Restore method re-creates these lost surfaces and reconnects them to their DirectDrawSurface object. Restoring a surface doesn't reload any bitmaps that may have existed in the surface prior to its loss. Therefore, if you lose a surface you must also completely reconstitute the graphics it once held.
For more information, see Setting Display Modes.