I built a basin out of concrete and it sunk in the middle do I cut out that section and then repour the concrete or do I start the whole thing from scratch?

Does the depression hurt or have any bad effects on the basin? If not leave it.
Maybe use bond-it and pour a cap on it?

Assume you are talking about a sink basin and not something to be used outdoors subject to freezing and thawing. If it is for outside use in cold climate, rebuild it.
Sounds like your mix contained too much water, which creates a high slump and weakens the concrete.
I agree with Strat's suggestions but I would also use a concrete sealant on the finished product in addition to the epoxy or other bonding material.
If you decide to start over, use only enough water to form a paste and coat all the aggregates. And, allow time for the bottom portion to take just about an initial set before pouring the top portion which can contain a little more water and easier to get a good finish. It's referred to as "rate of pour" in construction of deep structures. Good luck.