Not just the fan.
Make sure the metal block it is attached to (the heat sink) is properly mounted on the motherboard (not loose, falling off, etc.).´
The specifications of your computer and more details on your issue would be a good place to start.
Is the computer 100% stable when laying horizontally?
Do you get any errors before it crashes?
Is it overheating (GPU, CPU or anything else)?
Did you triple check that all power cables are plugged in properly?
Did you triple check that nothing else is loose/unplugged?
Does the computer still shut off if turned on but not in Windows?
Ummm I Didnt Check If The Metal Block is properly Mounted Or Not Will Check It now And The Specs Of My Pc Are
Intel Core 2 Duo E4500 @2.2Ghz,
Corsair XMS2 4 GB,
Western Digital HDD 500 GB 16MB Cache + Seagate 7200.11 1TB,
Cooler Master PSU Real Power M620W,
nVidia XFX GTX 275.
And Yes The Pc Is 100% Stable When Laying Horizontally
And I Dont Get Any Errors Before Crash
I Dont Think The CPU or GPU is over heating cause its working fine when placed horizontally
i will check the cables now
and i think i doesnt turns off when not in windows !