a debt collection agency has frozen my bank account. what do i do now.

In order for a debt collection agency to affect your bank account they need a court order; for them to get a court order there has to be a lawsuit. The collection agency probably sued you and got a judgment against you for the amount of the debt so the only way to "unfreeze" your account is to pay the debt or declare bankruptcy. If, however, the debt collection agency did not sue you and they improperly "froze" your account, it is possible that they are violating the Fair Debt Collection Practices Act and you are entitled to damages.