If you have good pressure elsewhere then you have a problem in a pipe. Could be a piece of trash was in a new pipe and lodged near the sink. Try removing the valves and letting the water rush out and maybe it will come thru. One thing that can cause this is if someone used too much solder and a blob formed INSIDE a fitting. It breaks loose and stops up someting down the line. Also if you have galvanized pipes they were probably restricted inside to begin with and then it doesn't take much.