No water in en suite cold tap

Please help. Built a house 4 years ago and last weekend the ballcock went in the tank in the attic. I turned off the water under the sink before I changed the ballcock...however when I went into the attic, the water was still coming in. Couldnt figure out what was wrong, so I went into the hotpress and closed all the valves to see would this stop the water going into the attic. This didn't work either.
Eventually i copped that I turned off the wrong valve...it was the outside tap. So I closed the proper valve under the sink, opened all the valves in the hotpress, changed the ballcock and turned back on the water. Everything perfect except there's only a dribble of water from the en suite cold tap.
I checked all the valves and Im sure I have them all the right way. Can anyone help...don't want to call a plumber!
Tried to explain this as best I could!

i would think 1 of 2 issues:

1 - you havnt turned back on a value; check them all one by one and confirm

2- you caused an air lock in the system.Easiest way to sort it out is to turn off the cold water again and flush the toilets. turn on the taps downstairs slightly and turn back on the water, you should hear the sound of any air travelling back up the system. turn off all the taps and your air issue should be sorted.

outside of that call a plumber......

The other well-known solution is connect a tube across from hot tap to cold tap (or with a mixer tap just block the end of it with your hand). Turn on hot tap to force air block in the cold supply back up the line.

Neither of these worked

call Mr Plumber