Can a creditor or debt collection agency access my bank account for debt collection?

Yes they can. After they have obtained a court judgment.

I fell behind on two payments on a court ordered payment plan and the creditor sent a letter asking me to contact them within 10 days. I did not. I am not sure how they were able to do it, but they got a court order placing a hold on my checking account (took all the money that was available) and I am not able to withdrawal any money from the account until such a time that the debt is satisfied. I would advice that you contact them to try and set up some sort of payment plan if and when they do get access to your account.