Yes, you can always sue. However, by the time you spend the time and money, they will probably reinstate your insurance if the money was there. It may be better, at this point, to engage an attorrney to deal with the entities who have made the errors in coverage. Most likely, it will be resolved with sevaral telephone calls once you give the attorney the written authority to discuss it with those entities. If that does not work, then suit should be filed.