Only wild speculation can answer your question without basic
information. Suspects start with power button switch,
motherboard power supply controller, and power supply.
Procedure to isolate reason for failure, in minutes, is
detailed in a previous discussion entitled "Computer doesn't
start at all" in alt.comp.hardware on 10 Jan 2004 or
http://tinyurl.com/2t69q
Which operating system? If using the reliable Windows NT
based OSes (NT, 2000, XP), then application programs would not
be on a 'reasons for failure' list. Furthermore, these
reliable OSes maintain a record in the system (event) logs.
What do logs say?
Additional reasons exist. This only starts to eliminate
some most common reasons. But again, first provide facts.
Only then can we discuss solutions. How to provide those
facts starts with the previous discussion and system logs.