It sounds like a motorized valve problem. If it was a gradual decline in hot water, it sounds like the motorized valve was working fine but then began not opening fully, getting worse each day opening less and less until finally it does not open at all. The valve in question should be located close to the hw cylinder, but may not be depending on how it was plumbed.
Check the motorized valve, unscrew the heat of the valve (no need to disconnect wiring) and you will expose the brass turn head. Turn this so that it is in line with the pipe, (perpendicular to the pipe is off). Turn on the boiler and you should feel heat passing the valve and heating the cylinder. It could also be sludge in the system that is blocking the valve from opening fully but more likely a faulty valve. You will know if you get any resistance when turning the valve manually.
To replace, just purchase a new head of the valve from anywhere such as Heatmerchants or Chadwicks. It will need to be the same brand and type of valve that is there already, otherwise you will need to replace all of the valve.
If not that, get back to us.