Billybob, it depends on the type of case. If a case has already been decided, then typically that's it. The term is res judicata, latin for "already decided." If the person can PROVE there was a fraud on the court, or have some basis to introduce new evidence (such as the DNA that frees people from time to time), then maybe they can get a technical "d over."
So if your lawyer just sucks, well, too bad. On the other hand, if your doctor sucks, cha-ching!!! Uncle JB needs a new pair of shoes!!!