If it's a '98 it has a freehub not a freewheel.
Freehub: The ratcheting mecanism is built into the hub. Sometimes it can be removed. The gears/cogs slide onto a freehub.
Freewheel: The ratcheting mechanism with gears/cogs attached is not part of the hub and screws om/off.
Sounds like the ratcheting mechanisn is either gummed/frozen or its broke. The process to un-gum it is similar:
Freehub: Remove gears/cogs, remove axle, liberaly apply some type of solvent to flush out the ratcheting mechanism. Lube. Re-assemble.
Freewheel: Can be done on bike. Aplly slovent to flush out the mechanis. Lube.