Windows 98 will not boot

windows 98 will not boot now. It comes up with a black screen that says disk error. When I try to go into Bios to select Safe Mode, it comes up with the same error. This happened when I was trying to uninstall Windows XP, and someone told me to go into the setup and change the Bootup Sequence from C to A. But now I don’t know how to change it back. Could this be my booting problem? Thanks