Hot water problem

Hi,

I am experiencing a loss of hot water fairly regularly these days. I have a hot water boiler which feeds three circuits of hot water baseboard in the house and heats hot water that is stored in a tank in the basement. Recently I have noticed that we often lose the hot water. When I turn the thermostat up enough to turn the boiler on for the baseboard heat, we will once again have hot water for a while. However, since the weather is warm enough now that we don't have the heat on normally, we keep running out of hot water. Is this a thermostat problem? Is there a thermostat in the hot water tank that should be calling for the boiler to kick on when it gets cool enough that is probably not working or could it be something else?

Thanks for your time,

GES145