sounds like its jammed,
unscrew the top plate (this might have a C on it)
if it dosnt have one or has a curved handle, pull it upwards
You will then find a nut, undo this. Use the longest spanner you have got as this is bound to be stuck!
Once it come off, unscrew the washer assembley, you may find this is a left-hand thread.
once this comes undone, then re-assemble it and re-insert it back into the tap
Ric, August 2008