Remember no PC details given so here's the usual.
1. PSU is old or not rated for the system. I.E. too small for the task.
2. Motherboard is old, has BAD CAPS (google that) and the issue only gets worse over time. Leaving the power on only accelerates the motherboard to "the end." Could be bad caps on other boards or even PSU but google BAD CAPS to see how to inspect for this issue. Still seeing such today.
3. Machine specific issues such as outdated BIOS, BIOS settings etc. Remember no PC details so I will write this even though I don't suspect it.
After normal shut down computer wont start,but after I pressed power button 20-30 times the computer normally turns on.It dont have any issues at all.I play games,go to the internet and everything works fine.I wont turn off the computer for 4-5 days now.Im afraid to turning it back off.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic