|If you have a standoff installed where there is no screw through the board then that can short out the board. Sometimes it doesn't happen right away.|
If you have a crappy case that has raised bosses instead of standoffs you should install insulated washers between the mounting plate and the board. Glue them on ahead of time.
Do you have an installed card reader or a combination floppy/card reader installed? If so disconnect it until after install.
Are you using onboard video or an add in card? If you aren't getting video it may be the card/chip.
Getting any beeps?
Disconnect ALL drives from the Board and power. remove any external hardware other than wired keyboard.