Bathroom Tap not working

If its an old tap sometimes the rubber stop becomes detached from the head and sticks shut. or the thread has gone and its not pulling the tap up. you can buy replacements at your local merchants cheaper than replacing the taps themselves.Take a flat head screwdriver and remove the top cap or a Phillips and unscrew the head. Pull the head off. You will be left with the valve in place. Isolate the water supply and remove the valve with a spanner. Take it and the head to your local plumbers merchants and they will supply you with a replacement. SIMPLES

Or you might have an airlock if you are on a gravity system.

The rubber washer inside the tap could possibly be stuck depending on the type of tap it is . If so , These can be repaired.

First of all, is the system mains fed or gravity fed? If it’s gravity fed, it’s likely to be air locked, the easiest way to fix this is to blow the air out via a mixer tap, or contact a plumber that will be able to rectify this issue. If it is mains fed, then it would be an issue to do with the tap cartridge