Hi @didierbonn64003
What EZ debug LED lights are turned on on the motherboard when you try to turn the PC on?
See p.28 (41 of 357 pdf) of the [https://download-2.msi.com/archive/mnu_exe/mb/MAGB550TOMAHAWK.pdf|user manual] to find their location on the motherboard and what each LED means.
If it's a ''boot'' failure, try the procedure as shown on p.25 (38 of 357 pdf) to ''clear the cmos'' and check if doing this allows the PC to turn on.