NEED HELP ASAP

Hey Guys/Girls!

Let me just start by saying that I'm absolutely hopeless when it comes to using a computer let alone doing anything else on one!!
So,
I'm borrowing my sisters old HP Pavilion dm1, it's been working perfectly fine since she got it, it's been working since I've borrowed it, only problem is that I closed the lid(turned off) yesterday and today I can't use it because it says Windows files may be corrupted?
I went into the BIOS settings via(F10), in hopes to move the internal drive to the top of the list, but it's not on the screen?!
I'm obviously assuming that it's not on the computer anymore then either?

Ps(sorry if my details aren't that great, I don't understand computers etc that well. All I know is, I'm in some trouble with my sis if I cannot fix this, thanks in advance anyway!!)