2002 Renault Laguna shuddered stopped and will not start

Car was driving faultlessly , a slight missing then total loss of power. Will not restart period. Cleaned diesel filter , there is diesel all the way to injectors. Have noticed that air seems to creep in somewhere but disapears when cranking. It sounds like it wants to go but just wont fire. The belts all look good. The only thing is i had some cowboys fit a gearbox twice "They messed it up 1st time" wondered if they could have done something , although it was running for a day after i got it back. Also now the voice computor has stopped talking to me.....