It's hard to say how much damage you've taken. Your hard drive could be failing. But it may not be that bad. For starters, do you have a Windows 98 Startup Disk? That's what might help you get the system going. If you don't have one, you can download one from Bootdisk.com
You probably want the 98SE OEM version. (That's assuming you have Windows 98SE - if you have a different operating system, find the disk for it. It can be important, but not so much for this next step.)
Start the computer with the floppy. It takes a while. When it's finished booting, you'll see a black screen with nothing on it but a A:\> prompt. Type:
. . . and hit enter. You'll now see a C:\> prompt. Type:
. . . just like that, with a space between the g and the /. You will be presented with five copies of the Windows registry and they will note whether they've started or not. Pick one that started and it will replace the registry. When it's finished you can try booting again and if it was just a registry corruption, it may restart. If a large section of the hard drive was corrupted, you may be out of luck and need to try something else.