My assumption is that your computer was working fine and dandy yesterday. I'm also assuming that your system's hardware configuration hasn't changed and that a virus is not the issue.
With that said, have you restored the master boot record (MBR)?
I had a similar case with Windows 98SE
In order to ensure any chance of this working at all though, you have be able to get to the A prompt first AND you'll need a reliable Windows 98 boot disk.
If you can get that down, put the boot diskin drive a: , and type "FDISK /mbr" and press enter. That's the command that will copy over the MBR from the boot disk to the C: (hard drive) drive.
Then you can try restarting the system (take the a: disk out first).
Other wise, if you can't even get prompts up...
Try resetting CMOS (go into settings and reset factory defaults, otherwise set it manually by the CMOS jumper on the motherboard - refer to motherboard manaul for assistance on this).
Reboot after this done.
If that doesn't work, check silly stuff like the ribbon cable connections from the drives to the motherboard's IDE / floppy ports, check the boot order in CMOS (as another poster pointed out before me), check power connections, etc.
Of course if the data isn't critical that's on the drive, you could nuke the drive and re-partition and that should definitely work unless its a motherboard/bios/cmos problem.
Hope this helps.
-Tom