The target remains restrained.
The text of the spell reads, in part,
When you cast this spell, and as your action for each round for the duration, you can exert your will on one creature or object you can see within range
If you can't see an object when taking your action, then you can't have an effect on it.
However, your effect from the previous round on the creature is still
Until the end of your next turn, the creature is restrained.
So here's how it would work out:
- On your turn you successfully restrain a creature until your next turn.
- On its turn, it teleports out of line of sight (but nothing says to end the "until your next turn" condition)
- On your turn, you cannot renew the hold as the opponent isn't an eligible target for you.
If you're looking for a similar effect, try Melf's Acid Arrow. The target takes damage up front, and also at the end of its next turn (regardless of whether the target is still targetable by the caster).