Hi there , heating not working without water being on but before fuse go next to the programmer it does work?

Hi there I have got the old system which has got tank in attic not combi boiler. problem is this one night fuse next to the programmer was blown and both hot water and heating were off I replace the fuse then hot water is working ok no problem but heating is only working if water is on if not heating is not working but before I was able to turn on hot water on time and heating 24hr now I can,t can any help me please