On modern digital cameras, the moving parts are generally motor-driven, not spring-driven. If power is lost mid-exposure, those parts usually stop wherever they are at that moment. On a Canon EOS DSLR, that typically means the mirror stays up, the shutter remains open, and the lens aperture stays stopped down until power is restored.

The interrupted exposure will not be completed or saved as a usable image.

This is generally not harmful. Cameras are designed to hold those positions during long exposures anyway. When power returns and the camera is turned back on, its startup routine normally checks the positions of the mirror, shutter, aperture, and focus system, then returns the camera to a normal ready state. Some cameras may show an error first, which is usually cleared by pressing the shutter or cycling power.

So the short version is: nothing should “snap back” mechanically, the shot is lost, and the camera usually recovers on restart.