I just installed a new 550W PSU after the old one stopped working, but now my PC won't boot. If I plug it in and press the power button, a few LED indicators on the motherboard turn on, and half a second later turn off. Then the power button ceases to have an effect; this only happens after unplugging and replugging. The lower indicator lights say everything is normal, and the upper first indicate all phases active, then the second, and then turn off. Could anyone help me?