The damage will remain 2d6 if the save is failed.

Where a spell effect calls for a saving throw, critical damage is not in play. Reviewing spells like lightning bolt, fireball, and thunderwave that have multiple dice for damage, nowhere does the spell effect describe a doubling of damage dice for critical hits.

Hit or miss, the shard then explodes. (p. 19 Elemental Evil Player's Companion)

What this indicates is that the explosion damage is not linked to the attack roll's success or failure, and thus cannot benefit from the auto hit or critical on a 20, nor the auto miss on a 1. (p. 194 PHB)

Ice knife is an unusual spell in that it has two separate effects. One effect is an attack roll (hit or miss, and thus eligible for a critical hit benefit), while the other effect is the standard save versus damage.

Compare to other elemental spells

As a point of comparison, see this last part of the spell description (p. 19 Elemental Evil Player's Companion):

At Higher Levels. When you cast this spell using a spell slot of 2nd level or higher, the cold damage increases by 1d6 for each slot level above 1st.

This is comparable to Lightning Bolt's similar "at higher levels" language. From PHB p. 255:

At Higher Levels. When you cast this spell using a spell slot of 4th level or higher, the damage increases by 1d6 for each slot level above 3rd.

That elemental spell effect is also influenced by a saving throw and has no "critical" hit feature.

Not as such, because the roll is never made.

Unlike the Bardic Inspiration, which modifies an existing roll (in this case a critical hit, the Portent die completely replaces the roll before it is made (PHB, p.116):

You must choose to do so before the roll, and you can replace a roll in this way only once per turn.

It does not negate a critical hit, because if a critical hit is rolled, then the roll has already occurred.

The Portent ability requires the player of the Diviner Wizard to be on his or her toes. Rolling combat can often occur very quickly after intent is announced (sometimes without intent being announced at all in a very relaxed game), so if the Diviner wants to use their Portent ability, they must be ready to announce it in that very brief time.

Note that many DMs will be lenient in these cases, especially where not much time or warning was given, and some may even be willing to skip back a second, and say that the Diviner just "replaced the roll before it happened" regardless of how it actually occurred at the table.

Finally, note that since the Portent die replaces the die rolled, a player who rolls a 1 or a 20 on their Portent rolls has a very powerful ability indeed, since they now can grant a critical hit or a guaranteed miss at will until their next long rest.