'''96 Litespeed Catalyst, '05 Litespeed Firenze, '06 Litespeed Tuscany, '12 Surly Pacer, All are 3x8,9 or 10. It is hilly around here!
The freehub pawls are not engaging. Eiither the freehub is really dirty inside or something is broken.
Try adding some light oil to the seam where the inner stationary "core" and the rotating outer shell meet to see if helps. If not, the freehub body has to be replaced. They are fairly inexpensive and don't require a new wheel.
At least, they're easy to replace if it's a Shimano hub (and possibly in the case of a Campagnolo as well). Various other hubs require special tools to remove the freehub body, and some can't be serviced at all.
If the hub is a shimano, and light oil (even ProLink or something like that) doesn't clear up stuff inside the freehub mechanism, and you need to replace the unit, a 10mm allen wrench loosens the bolt that holds the freehub to the main hub body. You'll need to remove the hub's axle in order to get in there.
The freewheeling mechanism is in either an old-fashined screw-on freewheel or a modern splined freehub.
Freehubs can be serviced but are usually replaced. You dont need to replace the whole hub.
Here are some illustrated instructions for repairing various freehub units.