If this is not one of the historically anti-IVA banks they could have done it in error and are proceeding as if your IVA is a bankruptcy --- I have seen it done ! If that is the case they should unfreeze the account when the error is pointed out.
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014 http://foggy.blogs.iva.co.uk