Sound's like the boot sector's been corrupted. You'll need a Windows 98 Startup Disk to fix it (or Windows 95 Startup Disk, depending on which you're running). Put this in the floppy drive and let it boot up. Once it's finished, type the following at the "A:\>" prompt.
SYS A:\ C:
This will recopy the core system files from the floppy disk to the special sector on the hard drive that the computer looks at to start loading the operating system (the "boot sector", enabling your computer to boot from the hard drive again.
After it reports "System transfered" and returns to the "A:\>" prompt, remove the disk and restart the computer.
Important!! Make sure that the Startup Disk was created on a computer running the same version of Windows that is on your machine, otherwise it may not work out!
To make a Startup Disk on another machine:
1) Navigate to Start Menu -> Settings -> Control Panel
2) Then open Add/Remove Programs.
3) Click the "Startup Disk" tab, put a blank floppy in the drive and click "Create Disk".
The process is pretty automated from there on.
If you've been hit by a virus or had some other more invasive problem (I'm suspicious, as this sort of thing doesn't usually happen out of the blue) then you may have more work to do, but this should at least make the hard drive bootable again.
Good luck!