A conjunctive phrase meaning "if that is the case" or "if/when it becomes so," used to introduce the resulting situation or consequence.
そうなると、別の方法を考えなければならない。
If that's the case, we'll have to think of another method.
彼が来ないそうなると予定を変えなければならない。
If he doesn't come, we'll have to change the plan.
情報が不足している。そうなると判断が難しくなる。
We lack information. In that case, making a decision becomes difficult.