It sounds like you’re developer abandoned you. Before you’re going to be able to work out what needs to be done to get the job finished the first thing you’re going to have to do is figure out where it’s at and the only way to do that is to pretty much figure out how everything was supposed to work, basically getting into the head of the developer that was doing the work.
Given this, your best hope is going to be to get the original dev to complete the project. If you can’t do that then to find a developer that’s willing to fix it.
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