Laptop won't reboot

Hello,
i have problem starting my laptop,it is an old siemens (manufactured in 2005) with window xp operating system. when i turn it on a sky-bleu screen comes up with two options f12 for reboot and f2 for BIOS set up.i press f12 and a black screen comes up saying operating sytem not foud.i press f2 and go through all the different setup step then when finish samething will come up operating system not foud.
would you please tell me what to do thank's.