On December 24, 2020, about 1440 central standard time, an experimental, amateur-built Sonex, N610DJ, was involved in an accident near Pecatonica, Illinois. The pilot sustained serious injuries. The airplane was operated as a Title 14 Code of Federal Regulations (CFR) Part 91 personal flight. The pilot did not provide a statement regarding the accident. Information from the Federal Aviation Administration indicated that, when the pilot reduced engine power to 2,000 rpm while on the downwind leg of the traffic pattern for landing, the engine lost total power. The airplane remained in a left turn while the pilot attempted to restart the engine, and the pilot stated that he lost control effectiveness during the turn. The airplane entered a stall/spin, then impacted terrain. The airplane sustained substantial damage to the fuselage and wings. Postaccident examination of the airplane revealed that the fuel tank was at least 1/3 full of fuel and was leaking from impact damage. The fuel lines to the engine were attached and secure. The engine could be rotated, and engine drive train and valve train continuity were confirmed. The carburetor bowl contained 100 low lead aviation fuel, and the carburetor float moved freely. The spark plugs electrodes exhibited normal coloration/features.