Highflow 400 Boiler not working?

Hi, I moved into a new flat with a Highflow 400 boiler about half a year ago. Its been working mostly fine in that time but has recently stopped working. All lights are on but no heating. I was told to reset the overheat thermostats but these seem not to push in or 'click' as i was told they would do. Thinking this may be a costy problem as I have no idea about boilers, as you may all have guessed!

Thanks,
Laura