Try isolating the incoming supply on boiler. You can use the water in the rest of the house then. Look under boiler and look for brass service valves. Post a pic and we can tell you which one. Might need a screwdriver or a spanner, some switch by hand
Ok all might be not lost ...you have turned. Your cold water supply off...you maybe ok to use your heating ..tell us what system. Expand on why you have had to close your water main off.
We are quite good on here ... centralheatking