Vokera Sabre 28 MTN combi boiler?

Boiler was working fine up to today, had a burst pipe (hot water to kitchen sink) (could still get hot water to rest of house) caused during cold snap in Dec which plumber capped off. Plumber reconnected it today but now my heating has stopped working, tries to fire up for a few secs then stops, green light just flashes on front of boiler. Can anyone advise please ?