Hi, the only reason I can think this happened is the script metadata got destroyed somehow, and the scripts connection is then lost. Perhaps something was moved from outside the interface (in the OS) or got deleted? If this happens, theres not much you can do, apart from "rewiring" all the sprites, as the script info has been lost.
Switch the inspector to debug mode, then drag the appropriate script into the script slot. Its tedious, but I don't think there is any other way to fix this unless you know exactly what caused it. Unity still stores all the relevant data, so nothing is really lost apart from the connection to the script.