... more than likely the jumper has stuck down onto the washer seat...
Turn the water supply off...
Separate the tap..fit a new washer .. it will be a 1/2'' washer...if you have access to a tap re-seating tool...skim the face.
Wipe all the threads with a smear of vaseline... and re-assemble.
Most old taps are always serviceable ... the new ones are nowhere near as good.
If you do have to change it ... they are only about £6.00
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