A desktop powers on but shows no display. What would you check first?

Check monitor power, input selection, cable, and the intended video output before assuming a failed motherboard. Confirm whether the system completes POST using documented lights, beep codes, or diagnostic displays. With safe shutdown and power isolation, inspect seating and required power connections according to vendor instructions. Distinguish a system with no POST from a working system with a display-path problem. Change one component at a time with known-compatible parts where possible, and use anti-static precautions. Do not open a power supply to investigate internally.