Nothing happens at all when pressing the power button? Make sure the case's front panel power connector is plugged in properly to the motherboard's power header and wasn't knocked loose (then again, your post seems to indicate that your motherboard has an onboard power button that you've tried).
But first make sure it isn't what Scott said. I've actually seen a couple of people mention making that mistake and go days without figuring it out.
Edit: I forgot to mention that beyond that there could be a short somewhere on the motherboard, but this is unlikely if all you did was simply add an SSD. If you remain at a complete loss and want to test the power supply itself with some fans and/or drives directly attached, then try the paper clip method.Edited by MSmith83 - 9/19/12 at 12:45pm