following f75, no power to boiler?

I was trying to sort a F75 problem with Ecotec 831,drained all the system, took front cover off boiler switched off power supply to check the pressure valve.
Did nothing else as i had to rush out, tried to switch power back on and nothing! checked 13 amp fuse, it had blown, replaced it, still nothing.
Internal 2 amp fuse still ok.
Would be SO grateful for any advice, 75 yr old mother getting cold!

Maxmind
June 2011