Take pictures immediately.
Take pictures and lots of them, and/ or video the entire project. Clearly, you want to document what work was or was not performed and when. If the matter gets to court, the visual evidence will show the story to the judge without much argument needed.
Immediately notify the contractor of your displeasure.
Immediately send a letter outlining events leading up to the walk-off and then all remaining issues via certified mail return-receipt (or overnight with signature via FEDEX, UPS or the post office) to any and all addresses for the contractor or builder. This letter should give reasonable and firm deadlines for action to be taken. Demand not only compliance with the construction agreement, but demand immediate communication. For example: "Please contact me within 24 hours of the receipt of this letter. Further, please be back on the job site within 48 hours and continue the agreed upon work. If you do not comply with these requests, I will be forced to expend additional sums to finish your work and then pursue action through the courts against you for this breach."
Make a decision about what you are going to do.
If you have not received a response, and have proof that the letter was received, you should promptly decide what you are going to do. Are you going to resend another letter, but with a harsher tone, or are you going to assume the builder is gone and go get estimates to have the remaining work done. Do not sit around hoping the contractor will magically appear.
Get estimates to complete the job.
If you get estimates, you should get three estimates for completing the remaining work. Try to avoid any additional or extra work being included in these estimates as it may seem that you are trying to get the first contractor to unfairly pay for extras. If you need extras, consider separate estimates. Get the repairing contractor to agree to testify should this matter go to court and to cooperate in case preparation.
Timely finish the new work.
If you hire a new contractor, try to get the work finished within a reasonable time. The longer you sit on your hands, the more likely some damage is going to occur to the job site.
How bad is it really?
Always consider the legal costs and time investment associated with pursuing a bad contractor in court. If it will cost $2,000.00 to fix, seriously consider whether it is worth your time and money.
Small claims on your own.
If the cost to repair is under $15,000.00, you may want to consider filing the action in magistrate court (small claims court). In magistrate court, it is a good idea to have a lawyer, but not always affordable due to the amount at stake.
Big time court-big time lawyer.
If the amount is over $15,000.00, you will have to file in state or superior court and it is recommended that you almost always have a lawyer.
Keep your documents in a safe place.
Do not leave your evidence in the leaky garage or in the back of your SUV. Get a folder and make sure that you have an additional copy in another location such as your lawyer, your office or on a computer. If you lose the contract, at least you have a copy.
Take a lawyer up on a free consultation.
Even if you do not end up hiring an attorney, you should at least meet with one that offers free consultations in person or over the phone and see what they think. Even if you do not hire the lawyer, you might learn something.