Hello there mate,
Are you sure that you've plugged in the front panel headers correctly? These are small thin wires going from the front panel (on button etc) to the motherboard. They have to go in a specific way and it's quite easy to get them a bit muddled up. (Your motherboard manual will show you where to plug them in. Just make sure you're careful and put them on the right pins).
This should let you use the power switch etc! If no luck, and they're definitely plugged in right... There may be a fault with the front panel connectors