URGENT-Help needed with central heating problem

[shoaghal]

I turned off gas supply before leaving on vacation. Now the central heating is not working. Gas supply is OK. How do I know what type of heating system I have? There's a big sealed cylinder with a dozen pipes running to it.I used to only turn on a switch to have hot water.
Would appreciate any help, I should be missing something very small.
Thanks

[htg engineer]

Is there a pilot light on the boiler ? that would of gone out if you turned off gas supply, it may take a while for the pilot to light as the gas pipes will be full of air, so its best to light the gas fire and any other gas appliances first.