Has someone altered the gas flow slide control setting which if they had moved it to the wrong side then all that would happen is the pilot would remain alight but no gas would flow to the main burner. If this is not the case then another likely cause could be a damaged or ruptured diaphram in the water control valve which is a very simple and economical repair by a suitably qualified service person familiar with this product.
My advice is it is time you had a service person give this unit a thorough check over knowing that 10 years is a reasonable life expectancy for these low entry heating units.
Selgas Services Ltd
Craftsman Gasfitters, Plumbers, Electrical Service Technicians