undersink tap

I have an outside tap which I plumbed in under the sink. It is the "screw into the pipe" variety and it worked fine until recently. It was switched of using the valve below the sink and when turned back on only a dribble of water is now produced when the outside tap is switched on. When the valve is turned fully on the water flow stops completely. I have completely dismantled and reassembled it to no avail. ??? My question is - is the valve gubbed and can I buy a new valve for this type of fitting or will I have to buy the whole kit again. Ta
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