Overwateringg

what could 1 large overwatering do to a plant in the seedling stage?

it could kill it, depends on how overwatered and for how long. Stop watering? VV

Over-watering does kill plants if the watering continues. If
this happens let it dry out and hope for the best. Use a fan near the
surface of the soil if you can

I doubt that over-watering one time could kill a plant, they're pretty resilient. Just let it dry out completely before you water it again. I over-watered one of mine until the leaves were yellow and droopy and managed to revive it by drying it out.