This rhetorical question is a rebuke from God and can be translated as a statement. Alternate translation: "Stop mourning that I rejected Saul from being king over Israel." (See: [[rc://en/ta/man/translate/figs-rquestion]])
The term "horn" was sometimes used to refer to a "flask" that was shaped like a horn and was used for holding water or oil. A flask of oil was used for anointing a king.