My boiler dosent come on i have no hot water but when i turn the heating on boiler works but still no hot water what should i do

IF you have a boiler with domestic hot water coil----it is controlled by the aquastat (AQS)---this usually is a 5" x 8" grey box mounted on the boiler---may be inside the cabinet----this controls the hot water when needed----with this type system the boiler is usually always on-heated----it operates by the standing temperature set---140 to 160 degree F (+or-)---when you turn the hot faucet on the water is already hot coming out of the coil---as the temp drops do to the cold water entering---the AQS senses the temp change and turn on the boiler to heat up (fire) for the hot water demand--- if this doesn't do this the AQS is faulty or needs readjustment----now another issue is that if this has been serviced/repaired and the AQS replaced but not with the correct AQS it will not function correctly for that system----now there are exotic boilers (Baxi is one name) that have electronic controls that do the same thing but no standing temp---only operates on demand (has no large volume of water--system only hold
+or- 1 to 2gal if that) with reheating its own water via heat exchanger to generate domestic how water---this is the best I can come with do to the limited info provided---good luck--- Happy Rabbit

Hot water is gravity fed through your pipes. When you turn the faucet on the water will flow through the pipes. If your hot water heater is natural gas driven relight the pilot. Same if oil fueled. If electrical, check fuse to see if it has blown or the circuit breaker has tripped. If either has tripped, replace it and if it blows again the heater has a serious failure, you might need to buy a new one. If pilot won't light on gas unit, try relighting after 5 minutes, could be a bad thermocouple.