reset a worcester 24i boiler?

Our boiler has recently stopped working after making weird high pitched noises whenever water ran, the boiler case feels hot but we aren't getting any hot water, it will run hot for a few seconds ...then back to freezing cold, under the cover I can't see a flame. On the top where they say the *reset button is, I cannot find, I only see a screw/bolt type one n one with a cap...help please !!