"I was busy at work" talks about the past. "I've been busy at work" talks about the present. This could mean the speaker is still at work and busy, or be an explanation for why s/he is tired, stressed or forgetful at that moment.

I've been busy at work ==> here the action started in the past and is still continuing now. I was busy at work ==> here the action started and finished at a specific time in the past.

one is a statement and the other is a Question

Thank u Amitaf & Peachey