Dentist's office closed mid Invisalign treatment. What should I do?

Sorry to hear for your trouble. however, don't disparage, there is a way out. The most important thing to do is to find a dentist who will be willing to take over the case either at a nominal fee, and some will be willing to take over the case knowing that you will turn into a regular patient that comes in for periodic cleanings and other Dental needs. Invisalign will gladly switch their records for you at your dentist request and will produce any missing aligners.
Personally, we have done this at our practice before without any trouble.
 Best of luck,

You can sue your dentist for abandonment. What he is doing is unethical. I usually do not criticize other doctors; however this simply wrong.

you can find another office that does Invisalign. Your case can be transferred to that office fairly easily. You would probably have a small fee to pay for the remaining  visits to monitor the case- but should not be close to the fee you paid for Invisalign