PSU or motherboard problem plz help

hey im having quite a big problem wondering if any of you fine people could help here i go,

ok so my computer refues to turn on my first thought is a power supply problem so hopefully the psu tester will be here tomo but just to see if im thinking right if the computer isn't turning on at all would i be right in thinking that it's either the motherboard or the PSU thats fail

thanks for your help and hope someone can give me a reply asap