If you have just re-furbished your computer say replacing parts (memory, video cards, hard drive and other bare essentials) make sure all power connections are properly seated and connected then try to power-up. If that fails, try to un-plug and re-plug all cards and connections (IDE cables, SATA Connectors, power supply connections, etc.)
connections to ensure they seat properly (it would be nice to check for bent pins before you re-plug them in their proper sockets for that sometimes happen) and check for broken pins that may be preventing proper connections. Re-check what you have done by powering up and see if that works.