Unfortunately, you are in a bad position. When it comes to "collections" i.e. collecting on debts owed the key concept is 'leverage' and you have very little leverage, if any.
First, you are not an employee. You are an independent contractor so the suggestion earlier about hiring an 'employment lawyer' isn't going to work for you. In the eyes of US law, you and your client are two business entities. So you would need to hire a lawyer who specializes in contracts.
Now, what do I mean by leverage? Well, assuming you win your case how do you collect your winnings? Will the sky open up and deliver your award to you? No. You have to use force. In this case, force takes the form of a court order which is delivered to a Sheriff who can then go to your clients bank and seize the award on your behalf. (The bank will comply with a court order but in the bizarre instance where it refused, the Sheriff could literally use force to seize your funds.) If there is no bank account, no funds or no information about that account you could place a lien against some other company asset but you might not get a recovery from that for years.
I have no idea what Pakistani law is like but if it is anything like US/English law, unless your client has assets in your country your courts won't be able to do anything for you. Thus, you will need to find an attorney who is licensed to practice in the state of Arizona or, perhaps, one licensed to practice before the federal courts in that area. These are questions of "jurisdiction." Since you are from outside the US, you may be able to sue in federal court.
Of course, you have the right to represent yourself. It would be foolish to do so, however. Unless you are fully educated in US legal practice your client's lawyers will run circles around you and run up your legal bills.
Your legal costs could get quite high and they might eat up or exceed to the amount you could recover. US laws usually require each side to pay their own costs. Only in special cases will the judge require the loser to pay the winner's legal fees. (This is the opposite of the English rule).
You should at least try to contact a lawyer and find out for certain what your chances of recovery are. It may be that your client is small and the mere threat of a lawsuit may make them pay up. Or it may be that they are large and it will cost them less to pay you than fight you.
In the end these have to be BUSINESS decisions for you. Don't get caught up in "justice" over a business contract. That kind of thinking can cost you a lot of money and stress. Calculate the risk and return on investment based on getting advice from a licensed professional.
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So, you may have lost a large amount of money but if you can weather it and continue contracting in this way there are things you can do to protect yourself in the future.
1) Get a written contract. Make sure YOU have an attorney who can make sure your contract is solid. You are dealing in international trade. "Gentlemen's agreements" are a great way to end up homeless.
2) Payment up front. You can do this in a couple of different ways. You can get a retainer for part of the work and renew the return for each phase. OR you can have the value of the entire contract placed in escrow which is held by a neutral party for payment when the contract is finished.
3) Break up the work. Never commit to a long duration/large amount of money. Work in monthly increments. Set milestones and bills every two weeks or monthly at the longest. If the client refuses to pay or gets difficult, stop work. If you do this then yo minimize the amount you can lose on a contract and you can find other work to mitigate your losses.
However you choose to operate the key to success is preventing risks like this. Trying to collect after you've taken a big risk is usually a losing strategy.
