My computer doesn't shut down nor sleep, either in Windows 8.1 nor Windows 7 (I have them both in dual boot). The power supply remains on so I have to press the power button for a while to shut it down. This didn't happen two weeks ago, I have been using the two operating systems without trouble for more than a year.
Motherboard: ASROCK Z77 PRO3
Processor: Intel core i5 3450 3.1Ghz
RAM: 2 modules of 4GB DDR3 /1600 KINGSTOM HYPERX KHX1600C9D381/4G
Hard disk 1: Seagate Barracuda 2TB 7200 RPM 64MB ST2000DM001-1CH164
Hard disk 2: the same
Power supply 700W 3GO Energy PS700SX (features on sticker: 3.3v—30A, 5v—32A, 12v—36A, 5SB—2.5A).
Motherboard: ASROCK Z77 PRO3
Processor: Intel core i5 3450 3.1Ghz
RAM: 2 modules of 4GB DDR3 /1600 KINGSTOM HYPERX KHX1600C9D381/4G
Hard disk 1: Seagate Barracuda 2TB 7200 RPM 64MB ST2000DM001-1CH164
Hard disk 2: the same
Power supply 700W 3GO Energy PS700SX (features on sticker: 3.3v—30A, 5v—32A, 12v—36A, 5SB—2.5A).