How do I get my old screensaver back?

I shut my computer down last night and when I restarted it, my old

screensaver was gone and replaced with something awful. I’ve shut

my computer off hundreds of times and when I restarted it, my

favorite screensaver always was there. Why has this suddenly

happened...and how do I go about retreiving my old screensaver? If

that can’t be done, how do I get something in my Photo library that

I can live with?