Why doesn't my laptop start?

I have a Lenovo ThinkPad and it was working fine, it used to be my cousin's and it worked perfectly. One day, I came home and turned it on and I left it there because I had to do something (I always did that. Nothing new) and I came back after a while and it was still in the windows starting screen. I thought it was weird but I said that maybe it was normal. I came back after a while again and it was still there so I turned it off from the button. And then I turned it back on and it still did the same. I talked to one of my cousins and he erased everything I had in the laptop thinking maybe it was a virus or something but it kept doing the same. He did some research and told me what it could be but he said he wasn't 100% sure about it. If someone knows what it is please tell me. I really need my computer for school and I don't wanna waste alot money.