Gas hob switch?

Hello
I have an indesit gas hob
One of the hobs has become difficult to swich on and off.

Its like its become jammed. However the cooker has been in use everyday for last five years and no problems untill now.

The problem is that one of the hob switch/dial
what you turn the gas on and off
has become stiff.....
extremely difficult to switch on/off.

It has always been kept clean
so i dont know what the problem is

please email me if you have a reccomendation.

Thanks
amy
handyam25yahoo.co.uk