I have a bathroom facet that has no water pressure and only has a small stream of water. I tried shutting the water off under the sink, but it still streams. I don't know what the problem is. What is causing this?

No pressure hot or cold ? Is this a single handle faucet ?

Remove the Aerator turn on the water see the water flow if it is good, clean and install the Aerator see the image below, good Luck.

John

If the aerator restores water flow post back and let us know GOOD LUCK, AC