hot water problem

hi again
had a problem with the hot water(none at all anywhere apart from school kitchen)
eventually found the cold water tanks that feed the plate heaters had run dry basically ball valve working as it shoud but not filling the tank as if there is a leak or a tap or taps left on or leak on the cold fill to plate heaters so isolated the feed to the plates which did the trick and tanks started to fill up.
so now how would i fill the hot water pipework? I know bout opening the cold fill and then starting on the ground floor and open taps one at a time and make my way upwards
forgot to say a firm was in to replace two boilers(old type cast iron section boilers)they said they did nothing to the hot water side just heating side but the cold fill was shut off so could this have drained the tanks because the building is occupied and in use as well as the kitchen just can't fathom how the tanks had drained or maybe a massive leak on the outlet side of the plate heaters but with such a leak it would have to show up somewhere no signs at all. The tanks hold a good 1000 litres or more

 

first assumption was an air lock did the normal stuff i was told to do closing the flow valve and return letting it vent through return that works always to no avail which made me think lack of water in the system but at the time i was looking at the wrong tank in the loft(cold water down service to cold taps)so then i checked clock tower tanks which was the problem.
checked valves all were on