Sometimes its best to cover the basics before calling out an engineer. This can apply to both boilers and fires.
SYSTEM PRESSURE ON COMBI BOILERS
If you have a combi boiler, you will notice a pressure gauge either on the appliance or just underneath. It will be a dial with usually a green section, some numbers and a pointer. when the boiler is cold and switched off, the pointer should be at the 1bar level or at the start of the green section. If the pressure is very low or at zero, then you will need to pressurise the system. Most combi boilers will have a silver braided hose linking two pipes together under the boiler. Each end of the silver braided hose will have a valve. turn these slowly until the pressure is at the 1bar level. Then make sure the valves are fully off. reset the appliance.
SOOT AROUND PILOT LIGHT
Gas fires can sometimes fail to light the pilot flame if there is a build up of soot around the spark electrode. Where you normally see the pilot flame, check the area for soot and debris. If possible, remove the soot and debris. It is possible the fire will require a service if this is the case.
HAVE YOU TRIED SWITCHING IT OFF AND ON AGAIN!
Sometimes a boiler can just lock itsef out over a small glitch. It is always worth trying to reset the boiler and seeing if it continues to lock out. sometimes, just a reset is all it needs.
REPLACE BATTERIES INSIDE FIRE
A lot of gas fires are remote controlled or have buttons on the side to turn them on and off. Most of these fires are actually battery operated and many customers dont realise there is a set of batteries inside the gas fire, not just the remote control. If you have access to the battery pack, it is always worth replacing these if the fire is not lighting properly.