Waht should we do?

We just close a house a month ago. A day or two we found a crack (about 3~4 inches long) in the foundation wall of the garage (interior wall). The crack is in the very hiding place, right close to the corner. And we did not find any crack inside the house. Is it a common thing for a old house (6 years old) or is it a big deal?

We did the home inspection by a profesional, he did not disclose that.

In this situation, what should we do? Whom should we go to ask for help?

Many thanks.