First, don't stress. This is normal. I've had a number of projects, including my current one, that has had early people come and go. A startup is a monumental commitment and it's a good thing your partner realized their heart wasn't in it before things got more serious (ie; product launch).

As for where to go from here, it depends a lot on the state of the current project IMO. If it's possible to get something out the door with the backend as it is now, I'd say do that. Call it an MVP, and see how it goes. If it turns out you're on to something, you can start looking for others to join you and use your traction as an argument. If it just absolutely doesn't work as-is, and you really believe in this, you should probably do what it takes to get an MVP out the door. That might be picking up the backend where he left it yourself, or hiring some freelance help if that's something you're more comfortable with -- be careful if you're new to this.

Best of luck!