how do i get my worcester 28i combi to fire up ?

How do I get my Worcester 28i boiler to fire up ? It went out a couple of weeks ago as the pressure dropped ( the temperature had been previously turned to lowest for weeks ) but managed to come back on several hours later after bringing the pressure back up .Water was coming from the outlet pipe whilst the pressure slowly dropped and it was suggested that the pressure valve had gone . I thought things had sorted themselves out as turning the temperature higher kept the pressure constant- until today - when it went out and wouldn't fire back up . I'm freezing my nuts off here , any suggestions please ? (the pressure is constant)