If your box was moved or bumped, check the monitor connections and be sure they are seated all the way in to the board and monitor. If you removed any internal connections, try re-installing them. If they weren't touched, then I would leave the power supply connections alone, except for the optical and hardrive connections. Double check those for something loose. The last option is static discharge damage to the board. While this is fairly rare, it's possible. I worked at Dell manufacturering, and we had to wear esd straps on our shoes to prevent this type of damage.