It sounds like the igniter has failed, is there a separate igniter button or is that part of the burner knob?
In some cases wate can get into the button and cause it to fail, but if the igniter is controlled by holding down the gas for each hob its most likely the igniter has failed\shorted out.
The fix rather depends if you have an igniter button or if its actuated as part of the hob control.
Since it may just be the switch and spray some WD40 may resolve it, as it will clean the dirty contact, failing that its likely to be the igniter itself which will need replaced, they are normally around £30 from online firms such as this one here.
With it being a gas appliance it is most likely going to need a gas safe engineer to attend to remove the hob cover, swap out the part and put it back together, if you would like me to recommend a suitable engineer in your area please provide your postcode.
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