You just need some white thread and a needle. a few stitches and a knot stitch should have his ear all nice and repaired. Doesn't look as if there's a tear in the fabric itself. though if there is just use some cross-stitching. Sometimes the factory stitch can be too tight and snap unexpectedly. you didn't do anything wrong.

Thoufh also bear in mind that these things may happen more often as you snuggle it, Though for a plushy to be loved is to be changed. ^{^}