Thanks for the suggestions (I have both versions too as need some of the functionality in v5 (easysocial etc.).
I followed the instruction from DAX. I installed latest version of 5, then I did the database integrity check and it fixed some tables, then I installed the latest version 6 again and it is working - no site crash this time. It did display this error after installing but guess it isn't an issue?
Error
Duplicate column name 'unsubscribe_total'
Duplicate column name 'unsubscribe'
Duplicate column name 'device'
Duplicate column name 'opened_with'
Duplicate column name 'mail_settings'
Sorry for my original post as was a bit stressed when it happened.
Mel