The quick checks worth making when the hot water stops, how to tell a simple reset from a real fault, and when it is urgent.
No hot water is sometimes a setting or a tripped switch, and sometimes a failed part. A few checks tell you which, and save a call out for something you could fix in a minute.
Check first
Is there heating but no hot water, or neither? Neither usually points at the boiler; one without the other narrows it down.
Is the boiler showing a fault code? Note it down, it tells the engineer where to start.
Has the pressure dropped below the normal band on the gauge?
For a hot water cylinder, has the immersion switch or its fuse tripped?
Is it only one tap? Then it is that tap or its supply, not the whole system.
Treat it as urgent if
There is a smell of gas: leave it, ventilate, and ring the National Gas Emergency Service on 0800 111 999.
The boiler is leaking water.
You have a vulnerable person or a young baby in the house and no way to heat water at all.
No hot water is listed under plumbing on Halo Hive, and marked as an emergency service so it is matched quickly.