No cold water to upstairs bath or toilet

Hi. Yesterday I needed to turn off my cold water (at main stop-cock) and hot water (at red wheel in airing cupboard) in order to replace a kitchen tap. For the hot water, I had to drain the system by running the hot taps for a short while. After the job I turned both cold and hot on again and all seemed fine. However this morning I've discovered that there is no cold water to the upstairs bathroom toilet or bath yet the bathroom basin cold tap is fine. Hot water is OK everywhere. I have a gravity-fed system so am guessing that the bathroom toilet & bath are fed from loft tank but the basin is mains-fed? I've no plumbing experience but am keen to resolve it without expense of plumber if possible, as feel it might be a simple solution with the right knowledge! Any help would be greatly appreciated! Thanks. Tim