motorised valve (2 or 3 port) may be knackered, this opens the valve in a number of ways to divert the water to hot water only, heating only or both on. It usually gets a feed from the programmer to switch it one way or another. Find it and check the feed to it .
If that feed is OK some motorized valves have a tiny lever that you can move into the right position to open the valve. If you do that and you then get hot water (After it heats up) it seems very likely you need a new valve or motor.
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