My wife and had a line of credit. The amount owing was $18000.00. I went bankrupt and of course the bank wanted full payment from my wife. We of cousre did not have the money so i made an agreement with the bank to pay $100.00 a month. All was good for almost 2 years
When we get a notice from a collection agency stating my account was past due and they are demanding immediate full payment. What can i do about this ? No notice was given from the bank. I have never missed a payment.

Have you started with calling anyone?

What you do is talk to the bank, of course. But it might be too late, if they sold the debt.
$100/mo is so small on 18K that it would take over 20 years to pay it off with interest - you could be long gone, divorced, dead. And it barely covers administrative costs for the bank handling the payments.
Plus, your wife didn't go bankrupt, and presumably she has a job? Perhaps the bank just found out more about your finances than you are saying.

First do you have the agreement for 100 dollars a month in writing ? If it is not in writing, they can merely say there is no agreement.

Next of course if YOU went bankrupt and then YOU agreed to pay them, ( not your wife agreed to pay them) this can cause a serious issue with other debtors in the bankruptcy if they find out you agreed to pay another debtor and not them.