"Losing any game at home is a massive disappointment but to lose by this margin is devastating," the Blues boss said.
"Any loss is tough to take but this one is very tough. We never started the game. We were lucky it was goalless at half-time in all honesty.
"It was a totally flat performance in every aspect. Barnsley played through us. It's an awful performance. There were lots of mistakes but everything we did was so flat. There was no urgency and it was lifeless.
"I said at half-time we had got away with it but we didn't heed the warning. We knew we hadn't performed.
"First and foremost, as the leader, I have to apologise to everyone associated with the football club for the performance. The players are an extension of me and try to mirror what I am about. But we looked as if we were disorganised and didn't play with any spirit. I have to take responsibility and the finger pointing starts with me.
"When it was 4-0, my wife was in tears. She knows what this job means to me. She knows how passionate I am about the game.
" I am totally committed to this job. This result is a kick in the stomach. I do not take any credit away from the opposition. I thought they were magnificent."