Why did I lose my water pressure after adding a new valve?

Hi,

I live in a row house where the water comes from one wall and continues to the next wall. The main valve supplies my home. From the main pipe in the basement, there were no valves to separate the powder room from the rest of the house. Only one pipe supplied the water to all the faucets.

I needed to renovate the powder room so, I cut the pipe and installed a valve to separate the powder room from the rest of the house. Since then, I have lost 90% of the pressure only at the sink on the second floor and, there is no pressure at all going to the powder room. And this only affects the cold water pipe, not the hot.

The basement sink works fine, the kitchen works fine, the shower and toilet upstairs all work fine.

Does anybody have a clue where my problem can be?

Thank you in advance for any replies.