My computer won't turn on!?

I have a Windows Vista HP Pavilion Entertainment PC. Recently, with no explanation, my computer just will not start. I take the battery out and put it back in and that works sometimes, but not always. What is wrong with this? I honestly can't think of a single reason. I don't know much about computers, so if you have a solution, please explain it in terms your 86-year-old grandma would understand. ;) Thanks.