Good news - the seedling will recover. It will take some time.
What I would do - use some sharp pruners and clip it right below the break so I get a clean sharp cut. I don't want ragged or torn.
You are focusing on the break - which is natural. I know the roots are an important part of the equation.
It will recover. You have to show patience and allow nature to repair and recover from the injury. Happens all the time in the woods, storms and wind breaks part of trees out. They continue to live and recover.
Eliminate whatever caused it to snap - rough handling or something dropped on it.
Bet you watch that seedling more than the others now.
That tree is wired to grow up. It will send a new bud out in the spring to compensate. Give it a year or two. If it wants to send up multiple shoots hoping to lead the tree into the next century, snip all but the best.
I planted a garden grown swamp white oak into my bottoms last March. I was sure proud of that tree. We flooded in April and along comes a beaver and saws it off. 10,000 trees in those bottoms and he finds my pride and joy swamp white. I saw it the other day and it has 4 shoots growing up now. I will prune 3 of them sometime this winter. No problem.