Hello:When a computer mysteriously does this, it's a BIOS issue.
You typically enter BIOS by restarting your computer and
immediately pressing the Delete key repeatedly until you
enter the BIOS Utility Screen (usually in blue). The fix
is fairly easy.
1. Once in BIOS, go to Power Options
(refer to the owner's manual if you do not know where it is).
2. Scroll down to Wake On LAN and/or Wake On Ring
and change them to Disable. This will prevent Scroll
down to Wake on LAN and / or Wake on Ring and Change
them to DISABLE
This will prevent the computer from being able to come
out of its power down mode whenever the phone rings or
if LAN activity occurs
3)Once on Bios go to to Power Management, then to Resume by PCI PME and disabled that as well
Press F10 and choose YES to save settings and exit. Restart your System.
Bon Courage..