Turn the water heater gas valve to pilot, push down the red button, place an open flame close to the pilot and it should light.
Sometimes people don’t always get everthing lined-up corrctly or the darn pilot lit is just hard to light.
On hard to light pilots I use my plumbers torch with a large cold flame to get where needed, applying for only a very short time. If you use this method be very careful not to have raw gas spilling out of the burner if you over heat the pilot.
Plumbill is right call a qualified serviceman to check it out – could simply be a blocked pilot injector then again it may be a little more serious than that – now’s the time to call in the experts. Remember gas in the wrong hands can be dangerous.
Selgas Services Ltd Craftsman Gasfitters, Plumbers, Electrical Service Technicians