Question
You design and build a MVP to market to your target customer base, and find one customer who has an immediate need, and enough motivation to spend time adopting (and paying) for the application.
You continue to market to other prospects but can't seem to get traction, but this one early adopter is engaged and agrees to sign a formal contract, with a bunch of customizations unique to their business.
You end up not with a product, but a business application specific to this one customer.
Do you kill the project? Support the app for this customer? Try to get them to buy the application and manage it in house?
Answer
YOU REJOICE!!!!! [dun dun dun dunnnnnnnnn]
Congrats. Take a deep breath then go sell your product to that big customer. That money will buy you enough time to figure out what to do next.
In the meanwhile, you can buy me a beer.