Delonghi oven flames do not stay on?

I've got a Delonghi Professional GSS601L - seven faithful years of cooking until last week! If I push, turn, and hold down the oven button, the ignition clicks okay, and the oven flames light okay. But as soon as I let go of the oven button, the flames go out. I've tried holding the oven button down for a whole minute, but still the flames go out...

Can anyone suggest what's wrong with my oven?

Any suggestions how I can fix the problem myself?

Thanks in advance...