My computer turns of automatically

Hello, I'm Biffy
I have a problem with my p.c
it turns off auto matically even if i'm on a website like bebo or facebook
and i'm not sure why
once it turns of you can't turn it back on for another hour or so.
Some times it even turns off when it's not on the internet
and i'm not quite sure why?
I really need to know if i should just get a new computer or fix it
This computer is only 2 years old and it's started turning off nearly every 40 minutes
Is it the websites i'm going on or is something wrong with my p.c
I really need to know

Thank you
i hope some one can help me