Plumbing?

Without more detailed information it’s difficult to give the most relevant advice. I’m going to answer based on the assumption that you have a gravity fed hot water system, not mains pressure system. If this has just happened out of the blue, it’s probably the internal workings of the tap that have failed and will need stripping down and inspecting.
However, I suspect that there has been some recent work done on your plumbing, maybe the hot water side of your system has been drained down recently? If this is the case, you have an airlock. The simplest way I have found of getting rid of an air lock is to open the bathroom hot tap, then go downstairs to the kitchen tap.if it is a mixer tap, hold you hand over the water outlet of the tap and open both hot and cold levers. The idea is to force mains pressure cold water back up the hot water supply pipe and blow the trapped air out of the system. It can take several attempts to achieve and you have to hold your hand in place for at least 30 seconds each time.

Rupert
February 2022