Presumably, as jourdain's reply also assumes, the tap in question is the older type which has a moving piston-like insert with a washer on it. That being the case, turn off the water supply to the tap and dismantle it - this very likely entails first taking away the "handle"/knob. You will then usually use a spanner to unscrew the core from the tap. This will reveal the waterway and, depending on the state of things, the washer will either be intact and attached to the plunger within the core or else detached and possibly partly disintegrated. Occasionally the structure will be such that the plunger is not a screw-type which moves up and down on threads but is free and is "activated" by the water pressure as it becomes free to do so on "opening". If it is stuck in the waterway and reluctant to move up on its own then you could have the effect you describe - this is likely to be caused by wear of the washer which now allows the plunger to get too far into the waterway and act rather like a stopper in a bottle. In both cases the solution is to fit a new washer of the correct size.