You can get information about the estate from the court's probate file and from the personal representatives of the estate if you are a beneficiary of the estate. You indicate the bank and attorney were designated as co-personal representatives of the estate, but do not indicate clearly that the first personal representative was no longer able or willing to serve. Thus, if there were an initial personal representative who qualified as executor, that person would also have information about the estate. If you take under your father's will, you are entitled to an account of the estate's assets.
As far as how long attorneys keep files, most attorney keep them longer than is required, and there are many attorneys who retain files seemingly in perpetuity.
Good luck to you.
Michael S. Haber is a New York attorney. As such, his responses to posted inquiries, such as the one above, are limited to his understanding of law in the jurisdiction in which he practices and not to any other jurisdiction. In addition, no response to any posted inquiry should be deemed to constitute legal advice, nor to constitute the existence of an attorney/client or other contractual or fiduciary relationship, inasmuch as legal advice can only be provided in circumstances in which the attorney is able to ask questions of the person seeking legal advice and to thus gather appropriate information.
Michigan law pretty much requires an attorney to keep records forever, unless there is an agreement otherwise. You can check on the probate file with the county probate court. In Wayne County, the records are available online, here: http://www.wcpc.us/
If it is another county, you would need to check with them personally, to determine what is going on. If you have additional questions or concerns, please feel free to post them so we can give you a more precise answer.
Client confidentiality may prevent the attorney from being able to share information in the file with you. If there was no probate estate, but your father owned real estate, you might find some leads in the real property records at the register of deeds. If you see how the real estate was transfered, and to whom, this might be a starting point for you.
If you believe there were assets that should have gone through a probate estate, but didn't, you might be able to open a probate estate, then list the assets in the inventory. Once you have done that, you could file suit against the persons in possession of such assets to turn them over to the probate estate. Then they would have to be distributed to the heirs, or devisees in the will. This type of approach might be speculative, and a law firm could ask for a substantial retainer to commence it.
Only if there has been an estate opened can you attack what a personal representative did in administering the estate.
This answer is not specific legal advise. No attorney client relationship has been established. It is general commentary on the question presented, without the benefit of a full disclosure of all relevant facts. Seek an in-person consultation with a licensed professional.