All hot water taps in the house not working. ?

Its been happening every once in a while and we leave the taps open for a few hours and we are fine usually. Today, it took a much longer time to get the water supply restored.

Since there was is no water coming on the hot water taps, but cold water is fine, I am thinking its not a boiler/heater issue... seems like some thing is preventing water from flowing out of the water boiler??? I dont have an idea as to how the plumbing works around the water boiler area....If any of you have any suggestions to troubleshoot this problem, do let me know.

Thanks!