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?
It seems like you have managed to move into the coveted market of Enterprise level SaaS, well done.
If your market can sustain a high enough monthly cost to keep using your software then you are set. If so, you have a template for each compnay to follow and also a proven track record to sell from. Now you just need to find more companies and sell them a custom application for their businesses and get them paying a monthly fee for security and support.
Unfortunately, if you are in a market where the businesses cannot afford a $2000/month+ application fee, you may need to step back and rethink where you have ended up.
Let me know if this helps or if you need further assistance.
Sell the technology to the large client and sell support/services on that technology. Start a new company with the original vision you started out with.
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.