The rules for bonus-action spells state:
You must use a bonus action on your turn to cast the spell, provided that you haven’t already taken a bonus action this turn. You can’t cast another spell during the same turn, except for a cantrip with a casting time of 1 action.
However, counterspell seems to interrupt the casting of a spell thus preventing it from completing and causing it to fail:
You attempt to interrupt a creature in the process of casting a spell. If the creature is casting a spell of 3rd level or lower, its spell fails and has no effect.
So, if indeed the spell was never completely cast, are you still restricted to casting a cantrip with a casting time of 1 action that turn?