Moterboard (by 2 users)

That green light indicates that the board is getting power. You should remove any excess components from the board, especially expansion cards like modems. Check to ensure the front panel header connections are properly connected (like the power switch, etc).

Lastly, ensure that all of the screws are in properly, so that the board isn't shorting out. As a troubleshooting step, you may try resetting the BIOS with the CMOS jumper to see whether a rogue setting may have caused the board not to power up.