Goal Achieved, Motivation Lost

Congratulations on reaching a major goal! You are right to try to figure this out without taking any rash actions. What is exactly is going on with your motivation would take an in depth exploration of your life. But I can say a few things about your experience. What is curious is that you seem to have little awareness of what's different in your new situation. You have had a change in motivation, a change in your feelings towards yourself, your work and your world. I suspect you are missing internal cues and having trouble talking about your feelings, perhaps even understanding yourself and your moods.
Especially when we reach a highly valued goal, there is a natural let down immediately after we reach it. Reaching that goal required a lot of effort and creative energy. While reaching the goal was exciting, within moments the intensity of the excitement wains. Then we start to recognize all the complications, work, and hassle that comes with achieving that goal.
The change in responsibilities from being the very best chef you could be to owning a restaurant is huge. I suspect there are additional duties that interfere with your enjoyment of cooking. Working for someone else produces a paycheck and a sense of security. Owning a resturant is one of the most volatile businesses. I would imagine that that stress would be quite overwhelming. I have to wonder how else your life has changed.
I think you may want to consider some counseling or perhaps some coaching. Clearly you have a changed situation, and your motivation has been redirected. I suspect some of the other unplanned changes is sapping your energy. Or maybe you just need a new goal to work towards, something that serves your goal to own a restaurant. Perhaps you want to pay off loans as quickly as possible. You maybe want to create some marketing opportunities in the form of special menu items, special promotions or special prices. Good luck with your new endeavor.