He said: "I lost my son a few years ago and I had been having quite a time of that.
"And after three years of getting a lot of support from the church and a lot of support from people - fans and family - I decided that it was OK to go back to work.
"Because I even thought of retiring at one point, because it just felt like too much."
John - who is making his acting return in new movie 'Savages' - admits he is still impressed by his "extraordinary" career achievements.
He added in an interview with 'BBC Breakfast': "'Grease!' and 'Saturday Night Fever' were the number one and two records in history until Michael Jackson's 'Thriller' came along.
"So for five years we held the crown for one and two! It was pretty extraordinary. And still I think that 'You're The One That I Want' [from 'Grease!'] is the biggest duet in history."