If you did the work and they have not paid you for it, you can sue them based on "breach of contract" for the money they owe you for that work--but only that money; no additional sums. The legal theory is there was an agreement (a "contract") between you, whether written or oral (unwritten); if you did your part and did the work, they are then legally obligated to do their part and pay you the agreed-upon sum(s). Suing in small claims court, as your own attorney ("pro se") is often a fast and cost effective option.