If pressing the power Button does nothing, even though the board reports recieving power, its likely a connection issue.
One way to know for sure is if your board has jumper switches, or onboard switches to bypass it entirely. Which it may not have.
As metioned, the ones marked LED will be for indicator lights. The PC will function correctly with or without them. They are not critical.
This is also dependent on the Case itself. though many have them, not all will. Some may only have the HD LED, the Power LED, Both, or none at all.
Take a close look at your front panel connectors. they look similar to this:
These are from the case, specifically the power switch etc, connecting to the motherboard.
The motherboard itself should have connections similar to this:
Keep in mind this is an example and may not be exactly what you will see.
The important thing is to look closely at the motherboard as these will be labled.
Match the wires (note the + and -) and make sure the connect at the appropriate place.
Also, double check that the long, 24pin main power connection is connected securly. Typically on the far right side of the motherboard.
And also, check the CPU power connection. This will likely be a small 4 pin (square) connector, or maybe even a 8pin. This is usually on the top left area of the motherboard, left of the main CPU area.
Any time you are working with changing anything inside the case, you should ALWAYS make sure the PSU is unplugged from the wall, flip the switch on the back, and then power up to drain any exceess power in the system before you touch anything.
That goes for everything. Changing RAM, reseating GPU, adding a sound card, or even reseating power connections.