Where has Lagertha gone?

I think she's still alive too. No doubt she'll make an appearance later at either an opportune or inopportune moment.

Unless this is supposed to create a potentiality for later on when they need to negotiate with Alfred and she becomes their "insider advocate" when Alfred is again put under pressure to delay or modify his promise to give over East Anglia to them.

That's an interesting thought and probably likely given their relationship.

The battle is never finished for Lagertha. :)

Lagertha talks to Bjorn after reappearing in England: "Hey, you'll never believe what I have discovered while being away. Look - this is a gun and here are some explosives. They increase your power in battle many fold. There are many more where these come from! This will ensure our place in England I am sure, regardless of the armies that might be sent against us."

:)

Indeed - she was pretty much in a deep state of shock as well as tapping into the vibe of Ragnar during the time she was lost in her mental tempest.

She was lucky that Judith was seeing the healer or else she may have been gone for some time perhaps without the help of those who knew her.