How can I fix cold water to bathroom?

Hi, my little girl left a cold water tap running in a sink without me realising until problem with water supply. The tank in attic had emptied. We turned tap off and tank began filling again. Cold water pressure is fine downstairs but slowed to a trickle in bathroom, hot water is fine. I have no idea what to check or look for, to resolve problem and can't afford to call out a plumber. Any ideas? I may have a 2nd smaller tank up there too but have only looked at big tank with a large float in it. Thanks for taking the time to read :-)