If it's all of your hot taps then the cause is a blockage, which could be either air or debris.
Assuming for the moment that you have a gravity-fed hot water system, then start by having a look in your cold storage cistern (commonly in the loft) - the cistern should have a lid, and the water should be crystal clear.
If all OK there, then have a look at the any valves you find between the CSS and the hot water cylinder; they should all be gate valves, but if any is a stop cock then it could be harbouring a blockage.
Whether or it's air or debris you could use the same method to attempt clearance, which is to couple up the cold mains to a hot outlet.
It's easiest if you happen to have bath filler fed by cold at mains pressure - you just do this:
1. Put your palm under the spout to stop all water emerging.
2. Open the hot tap fully.
3. Open the cold tap partially, for about 10 seconds or so.
4. Close the cold tap.
5. Suddenly remove your palm from the spout (downwards to prevent cuts).
If you can't do it with a bath filler, then use washing machine connections (if you have both hold and cold supplies there) - just couple up one of the appliance hoses to cross connect the hot and cold valves, then:
1. Open cold valve.
2. Open hot valve for 10 seconds or so.
3. Close hot valve.
4. Close cold valve.
5. Open a hot tap somewhere to see if it's cured.
If these don't work for you then come back and seek more advice.
