With no model number it is impossible to be precise. First check the easy (and cheap) possibilities. The agitator has small plastic pawls (dogs) inside the upper agitator half that ratchet around the notched ring on the lower agitator half. You can see these if you pull the fabric softener cup, remove the agitator retainer bolt and lift off the upper agitator assembly. Agitator dogs (a set of 4) should be $4 to $6. (This assumes your model has a two piece agitator.)

Otherwise, is is likely the neutral assembly is broken or stuck. A small part, but difficult to access - located between the outer tub and transmission top. This type of repair is beyond many folks having limited tools and knowledge.