How can I earn the level 70 Kerrigan Power achievement if I wasn't awarded it?

Like Blem indicates in his comment when Kerrigan reached level 70 I did not get Kerrigan Power achievement. I was connected to the internet at the time of the level up and have not used any cheat codes.

Do I need to replay the campaign to earn the achievement? Is there a reason the achievement wouldn't have been earned even though Kerrigan has reached level to 70?

Solution 1:

Recently Blizzard released patch for those who earned level 70 for Kerrigan but didn't get the achievement. That will require re-enterring storey mode or Master Archives. They also promised to enhance bug-fix even more.

The Story Mode achievement Kerrigan Power: 70 is now awarded upon re-entering story mode or the Master Archives for players who previously reached Kerrigan level 70. (An additional component to this fix will be applied in a future patch, which will properly grant credit immediately upon fulfilling achievement requirements.)

Read in full here: http://us.battle.net/sc2/en/blog/9096527/

Solution 2:

The Kerrigan Power achievements (and quite a few other Kerrigan Achievements) are currently bugged (scroll down to Heart Campaign). The level 70 achievement will not be granted if you get the final levels from the archive.

Workaround if you have a save game from the Conviction mission(not the Archive).

1. Load the mission and quit it, this resets the campaign back to that point of the game.
2. Go to the Archives.
3. Complete any missed objectives to get Kerrigan to level 59.
4. Once you complete all of the Kerrigan levels, exit out of the Archives and go back to the Leviathan.
5. Start the mission Conviction.
6. Finish the bonus objectives within the level and complete it.