You may have several options. If you are a consumer, you may have the protection of consumer protection laws such as the Pennsylvania Unfair Trade Practices and Consumer Protection Law and the Home Improvement Consumer Protection Act ("HICPA").
Assuming the contract was for work at your residence and considered a "home improvement", you may be able to avail yourself of the protections of HICPA. HICPA applies to "Home Improvement Contracts". It requires contractors to register with the attorney general (is your registered?), requires the use of written contracts for home improvement of more than a few hundred dollars, limits the amount of a deposit a contractor can require, and prohibits taking a deposit (other than for special order materials) before a contract is signed. It also provides remedies for contractors who don't maintain start/end dates that they agree to in the contract. Additionally, home improvement contracts must comply with HICPA's strict requirements, and if they don't, the homeowner can void the contract. HICPA is enforced by the attorney general, but you can file lawsuits for breaches of HICPA as well. Check out the attorney general's site for more info: http://www.attorneygeneral.gov/hiccon.aspx
Additionally, fraudulent or deceptive practices against consumers are prohibited by the Pennsylvania Unfair Trade Practices and Consumer Protection Law . You may have remedies under this law as well.
If you are not a consumer or the work was not a home improvement, your remedies are likely more contractual in nature. If you can't persuade the contractor to return your deposit, this means you'll likely have to commence a lawsuit to seek the return of your deposit.