I assume this is on a Jeep (of some sort).  But we would need some
additional info e.g. number of miles on the vehicle, model, etc.  Wheel
bearings can go bad for a number of reasons. Typically the wheel grease gets
contaminated (water, dirt, wrong type of grease) or it may dry out (Brakes
get too hot from a stuck Brake) and the bearings wear out.

Nothing lasts forever (except taxes)

Daimler uses puny sealed ball bearings for the front like the Japanese
front wheel drive ricers:  http://www.billhughes.com/Dana30hub.jpg Repair
what you have:
http://jeep.off-road.com/jeep/article/articleDetail.jsp?id=257794 Real Jeep
hub conversion kit: http://www.4x4wire.com/jeep/tech/axle/yjwarnaxle/
       God Bless America, Bill O|||||||O
       mailto:BillHughes@billhughes.com
 http://www.billhughes.com/Dana30hub.jpg

They do just wear out and DC decided that wheel bearings were just too
damn cheap and easy to repack and replace so they made the new ones all
sealed units so you have to buy a whole hub.

Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
'New' frame in the works for '08.  Some Canadian Bush Trip and Build
Photos: http://mikeromainjeeptrips.shutterfly.com