If they haven't paid you for 7 months, and aren't returning your phone calls nor emails, they probably aren't planning on paying you.
If it's not a lot of money, forget about it, tell them you aren't doing anymore work for them, stay on top of things better next time, and move on.
If it's an amount of money that justifies chasing, I have had luck showing up physically at people's businesses. If you can't do that, I guess a lawyer is your only option.
Not to be a downer... but you probably aren't ever going to see this money. They may not have it. Maybe you can offer to take some of it a bit at a time rather than wopping them with a thousands of dollar bill at once? But be careful about doing any more work of them of course.
So I am guessing that this is a side thing and not your primary business. Because 7 months of non-payment would easily equate to $100k+ at reasonable rates. Missing that kind of income would be pretty obvious for a small team let alone 1 person.
Given that, my bet is the amount is relatively small and not worth litigating or really starting a war over. Don't post publicly on Facebook, Twitter etc, it does you no good but people/companies will see and remember it. Most usually not in the positive way.
Unless the amount is significant just write it off. I get when you need cash that this can be a hard thing to do, but it is the most likely outcome anyway, so put your efforts into finding a new source of funds.
However, that said, after you get it in your head that you aren't getting paid (changes you mindset), then try this. Send a personal email, voicemail and text saying something like: "hey look, I worked for 7 months for you and yea, I'm an idiot and didn't stay on top of my invoices, but man, I am hurting and could use the payment like we agreed." And say you are willing to take it in some smaller chunks instead of one lump sum if that helps his team out.
Your friend may have initially just screwed up too, and now be embarrassed that he got so far behind. He may have wanted to pay you, then realized the sum is larger now then he has free cash. But if you offer him a hand and a graceful way out, he may surprise you. At worst, he doesn't answer and you haven't lost anything more than a little time to write a note and make a call etc.
If you're short on cash, you should probably be looking for a client that will actually pay their bills. Tell your "friend" that your work on the project will stop until the work that's been completed has been paid for.
What else you can legally do depends on the terms of your contract. To protect yourself, you probably shouldn't take any action until you've talked with a lawyer. You don't have to bring an expensive lawsuit against them - just pay the lawyer for an hour to review the contract and advise you of your legal rights and responsibilities.
Don't revert the code. Even if you don't "break" anything, he could make an argument that you caused damages, and he could sue you. Furthermore, damaging his business isn't going to make him richer and more likely to pay.
You could, of course, say something like this:
"Because you have failed to pay me for my work, I feel it would be morally justified for me to revert all of my changes unless we reach an agreement in the next 10 days."
You're not actually saying you're going to do it. You're just saying you think it would be ok.
I'm not a lawyer, but on a personal level, this article has always resonated with me: http://philip.greenspun.com/politics/litigation/philosophy.h...
If you invoiced according to the contract and weren't paid and are not receiving replies to communications your friend is a deadbeat. I suppose you could lawyer up. That's a medium term strategy. The short term should be based on the assumption that you will never be paid. Some people are just assholes.
The way to reduce this sort of risk is screening clients and requiring retainers applicable against final invoice. And no favors for friends. I mean the reciprocal of you lowering your rate or being flexible on terms is your friend paying more than the standard rate under terms more favorable to you. Somehow that is not where people usually come from.
Don't be anyone's bank.
Good luck.
You might find a lawyer who will work on contingency. This is probably small time but you might find a young attorney to write some stern letters.
Also each state has a set amount of interest that applies to unpaid invoices if no other fees or interest have previously been agreed to. Look up the statutory interest and start computing and invoicing your late fees.
It's tough to know how much pressure to apply. You don't want anything to blow up in your face. For 7 months of unpaid work, it's a big enough deal that a lawyer could really help.
How close is the friend in question? I would not assume malice outright. I advise to check if your friend is alright, he might be sick or in jail.
Absolutely bring in a lawyer. Reverting the code is a bad idea because a) unless they're clueless they have made backups and b) that could allow them to sue you.
You are fortunate in that you have an actual contract and a paper trail, which puts you far ahead of most claimants in 'I got exploited' situations. Organize everything and bring it to a lawyer, who will explain your options and then offer to write a demand letter on your behalf. If the matter is as clear-cut as you say, then you should be able to ind a lawyer who will handle it on contingency. If you don't have a lawyer, call your state or country's Bar Association and tell them that you have a contractual dispute involving non-payment in the amount of $xxxx, and they'll recommend some lawyer sin your area that arelikely to handle disputes of that scale. Most lawyers will give you a free consultation.
Of course, look through your contract carefully first - it might include a clause that requires the matter to go to mediation or arbitration. The more familiar you are with your contractual situation and the better organized your paper, the faster and cheaper resolution will be.