operating system not found

i need a lot off help, so if anyone can help it would be greatly appreciated.
i am not a big computer user but i do use my computer, recently my brother tried to install a new version of windows on my computer, i used to have windows 95, and he was trying to upgrade to windows 2000, well in the process something got screwed up, and now it gives me operating system not found when i try to start it up, and now i cant even reinstall 98. can someone tell me what's going on and how, if possible, can i fix my computer.