Are you trying to boot directly from the CD-ROM? If so, it may be worth creating or downloading a Windows 98 boot disk and booting from that with the option to load CD-ROM drivers. If upon inserting the CD the system still won't run the disc, you may have a hardware problem.
It's hard to narrow it down without more information, but the first thing to check is the IDE cable. Failing that, the drive may be malfunctioning or it is even possible the controller is damaged - these are unlikely but all possible sources of problems.