this has been happening for a while with my 11 year old computer and it's scaring me a little but there are some times when this happen

for example,i remember i turned it on and went to take a shower for about 40 minutes(i take long showers)and when i come back it's off completely

no one could be shutting if off because i have no one in my house other then me.when i check the power save setting it's not set to shut off after 40 minutes i have it set to 2 hours