heya redss;
Since you didnt say what version of BIOS you have i will use Award as an example.
GO into the [Power] Tab and select [power up control] then select [AC power loss restart] set it to "Disable" or "previous state".
if it is disabled then on power loss the computer will not turn on.
if it is on previous state then it will return to the previous state it was at before the power loss, if it was off it will remain off, if it was on it will boot to the OS.
XeeN