No cold water downstairs?

I was wondering if anyone can help? I have cold water everywhere including the downstairs toilet with the exception of the kitchen sink. The Hot water is not affected and the boiler is fine but the pressure did drop yesterday but I refilled the loop and the pressure is now about 2 bar. This did temporarily restore the kitchen tap cold water supply but its gone off again. The outside tap which is linked to the kitchen plumbing is working fine. Any ideas please?