Hello, I just got done putting together my second build a couple hours ago. Well, I turn the thing on and after about 10-15 seconds start to smell burning, followed by the computer shutting itself off.
I opened the computer afterwards and inspected everything but saw no visible signs of damage. From inspecting the wires I noticed that the wires that connect the Case's power button to the motherboard were reversed, meaning the negative (white) wire was where the colored wire should have been and vice versa. I fixed this and attempted to boot the computer again but it wont stay powered for more than a few seconds. In fact it wont turn on at all unless I flip the switch on the PSU off and then back on.
If you have a power supply tester, use that to test the PSU. If that's okay, then I would say you just destroyed your motherboard.
I have never had the positive and negative power in the wrong spots (reversed polarity), and I don't know enough about electrical engineering to say whether that could kill your motherboard, but it sounds like it did.
A quick Google search shows that people seem to destroy things when the polarity of anything is reversed.