You're gonna have to burn it and start again mate - now that 1 root has come out the bottom of the growbag it'll be glowing like a fucking lighthouse on police thermal imaging cameras! I'll be surprised if you're not being busted as I'm typing!
Lol!
The non-flippant answer, is not much tbh dude! This is because; (a) you've already just transplanted it, and (b) the lead tap root will always pop out bottom relatively quick, as much for orientation as well as water searching, and hopefully lateral root growth should be occuring alongside it. If you managed to build up a good solid-ish rootball at cup stage then it should be enough to let it laterally expand ball out, rather than grow just loads of straggly single downers. If not, next grow just keep it in the cup longer, wind train your seedlings and water more lightly to encourage rootball growth. (Oh, and I've never used any root boosters though some formulas do seem to work magic tbf so could be cheap option to add next time too if roots really do go south on this one! Lol sorry!)
You do say it in a bag though, so depending on viability you could have a Google of how airpots work as may be able to just mod your bag up - I can't remember whether it's just straight holes in sides or whether they slightly concave for optimum function, etc though a few stabbed straight through may air prune slightly if they not reached those areas yet - have a check out as I honestly can't remember atm! Overall, though dude I mean I wouldn't worry too much atm - it is good to check root size and health if you are ever concerned about issues though.