In many states an attorney is allowed to assert a lien on a file for unpaid fees. How much is a fair fee? Your state bar may have a free fee dispute committee. Alternatively, the judge on your case may, upon a proper motion, decide how much your former attorney is entitled to be paid before releasing the file.
If you are seeking other counsel, hav eyour new attorney contact the previous counsel and they will probably be able to work it out to transfer the file. If not, the new counsel will be able to advise you of best remedy to obtain the file.