switchmaster 300?

i have a switchmaster 300 about 14 years old.the timer has stopped and will not restart.i have had this happen in the past where it has stopped after a longish power cut,and i have got it going by turning the clockface dial around a few full turns,and it has started ticking again,but this time there has been no loss of power,and i cant get it started again.the boiler works ok when on constant thanking you