If you haven’t been using any of the valves inside ( have you?), then chances are you’ll need a plumber to fix the issue, so you may as well just get one ASAP to diagnose it as well.
Having said that, if you want to try and figure it out, I’d start from the ‘top’ of the system. So the mains water is obviously coming into the house as the kitchen tap works. The other cold taps though will be fed from a big tank of water in your attic (or hot press in an apartment). This tank is filled from the mains incoming water, so first thing is to check if that tank is empty or full. There’s a valve that lets water into it when the level of water drops, it could be stuck shut.