Hi
What do you mean not booting up?
Does any thing appear on screen?
Is there any appearance of windows starting up?
(Assuming windows of some sort)
If PC is completely dead :- no noise , no LEDs , display check fuse if in plug or fuse box
Check power cable connector properly in back of psu and psu switch on
Next check psu . This needs a spare psu of suitable rating or a psu tester
Mother board do fail especially if capacitors fail but usually failure is intermittent
If problem start when windows loads try booting off bootable CD, DVD or USB disk.
Start up repair may work if windows is corrupted or chkdsk c: needed
Regards
Mike Barnes